refactor: comments state current behavior, not project history

Comments across the tree justified the code by pointing at a Rust
implementation that is no longer in the repository, citing files and line
numbers (config.rs:132, schema.rs:26-54, reader.rs:42) that cannot be
opened, plus crates and datasets that are equally gone. A reader could not
check any of it.

Every invariant those comments carried is kept and restated so it stands on
its own: the bytewise sort that decides which row survives a duplicate exam
number, the literal U+0300..U+036F range that must match the site's toAscii,
the trailing space in "SINH ", the BIFF and shared-string corrections, the
VACUUM-after-COMMIT rule, the deploy-from-main guard.

The reader's contract is now anchored to the frozen oracle in
parser/testdata, which still exists and is still checked, rather than to the
tool that originally produced it.

TestDDLMatchesRust becomes TestDDLIsFrozen: it compares against a copy of
the DDL inside the test and never read schema.rs, so both the name and the
failure message were misleading.

ToAscii no longer claims the d-replacement must precede lowercasing. Both
cases map to 'd' and ToLower runs last, so the order has no effect.

parser/internal/transform/transform.go

@@ -1,9 +1,5 @@
 // Package transform performs row transformation: ASCII normalisation, score
-// regex parsing, and validation — a port of parser/src/transform.rs.
-//
-// ToAscii replicates build-lib.js toAscii exactly:
-//
-//	str.normalize("NFD").replace(/[̀-ͯ]/g,"").replace(/đ/gi,"d").toLowerCase()
+// regex parsing, and validation.
 package transform
 
 import (
@@ -26,14 +22,11 @@ import (
 //  3. Replace đ/Đ with d (NFD does not decompose them)
 //  4. Lowercase
 //
-// Step 2 filters a LITERAL CODEPOINT RANGE, not a Unicode category. The inline
-// comment at transform.rs:53 says "Unicode category M", but the code at :56 is
-// the specification and it checks '\u{0300}'..='\u{036f}'. unicode.Is(unicode.Mn, r)
-// is strictly broader and would strip marks Rust keeps, silently changing
-// ho_ten_ascii — the column the site's accent-insensitive search runs on.
-//
-// Step order matters and mirrors transform.rs:54-63: the đ/Đ replacement happens
-// before lowercasing.
+// Step 2 filters a LITERAL CODEPOINT RANGE, not a Unicode category.
+// unicode.Is(unicode.Mn, r) is strictly broader and would strip marks this keeps,
+// silently changing ho_ten_ascii — the column the site's accent-insensitive
+// search runs on. That search normalises the query with the same four steps in
+// web/src/App.jsx toAscii; the two must produce identical output or lookups miss.
 func ToAscii(s string) string {
 	if s == "" {
 		return ""
@@ -44,7 +37,7 @@ func ToAscii(s string) string {
 	b.Grow(len(decomposed))
 	for _, r := range decomposed {
 		if r >= 0x0300 && r <= 0x036F {
-			continue // combining mark, in the range Rust drops
+			continue // combining mark inside the dropped range
 		}
 		switch r {
 		case 'đ', 'Đ':
@@ -73,23 +66,22 @@ type ParsedRow struct {
 
 // SkipReason says why a row was skipped, or SkipNone when it passed.
 //
-// The distinction is load-bearing for the printed counters, and the two
-// non-blank reasons are counted as source rows while BlankRow is not — but note
-// that split lives in the CALLER, not here. parser/src/main.rs has two call
-// sites with opposite outcomes for BlankRow: at :135-137 it returns before the
-// counter at :140, while at :151 it matches Err(BlankRow) => {} and falls
-// through to transform and insert. The build loop must reproduce both.
+// The distinction is load-bearing for the printed counters: the two non-blank
+// reasons count as source rows while SkipBlankRow does not. That split lives in
+// the CALLER, not here — the build loop drops blank rows before the source-row
+// counter and, at the later switch, lets SkipBlankRow fall through to transform
+// and insert. Both call sites have to stay as they are.
 type SkipReason int
 
 const (
 	SkipNone SkipReason = iota
-	// SkipBlankRow: row is fully blank (2017-old2 only, checked before the
-	// source-row counter).
+	// SkipBlankRow: row is fully blank, and strip_blank_rows is on. Checked
+	// before the source-row counter.
 	SkipBlankRow
 	// SkipEmptyField: so_bao_danh or ho_ten empty/missing.
 	SkipEmptyField
-	// SkipNonNumericSbd: so_bao_danh contains non-digit characters
-	// (2017-old / 2017-old2 guard).
+	// SkipNonNumericSbd: so_bao_danh contains non-digit characters, and
+	// require_numeric_sbd is on.
 	SkipNonNumericSbd
 )
 
@@ -109,10 +101,10 @@ func (s SkipReason) String() string {
 
 // ValidateRow checks a row against the dataset's validation rules.
 //
-// Signature mirrors transform.rs:101-107, taking stripBlankRows and allBlank
-// explicitly; a shorter signature could not express both blank-row paths.
+// stripBlankRows and allBlank are passed explicitly because a shorter signature
+// could not express both blank-row paths.
 func ValidateRow(hoTen, soBaoDanh string, cfg *config.ValidationCfg, stripBlankRows, allBlank bool) SkipReason {
-	// 2017-old2: skip fully blank rows BEFORE counting source rows.
+	// Skip fully blank rows BEFORE counting source rows.
 	if stripBlankRows && allBlank {
 		return SkipBlankRow
 	}
@@ -128,11 +120,11 @@ func ValidateRow(hoTen, soBaoDanh string, cfg *config.ValidationCfg, stripBlankR
 	return SkipNone
 }
 
-// allASCIIDigits mirrors Rust's chars().all(|c| c.is_ascii_digit()).
+// allASCIIDigits reports whether every byte of s is an ASCII digit.
 //
 // Deliberately not strconv.Atoi: Atoi accepts a leading sign, so "+123" would
-// pass a check Rust rejects. Empty input returns true, matching Rust's all() on
-// an empty iterator — the empty case is caught earlier by RequireNonemptySbd.
+// pass a check that must reject it. Empty input returns true; the empty case is
+// caught earlier by RequireNonemptySbd.
 func allASCIIDigits(s string) bool {
 	for i := 0; i < len(s); i++ {
 		if s[i] < '0' || s[i] > '9' {
@@ -146,7 +138,7 @@ func allASCIIDigits(s string) bool {
 //
 // Every one of the 16 patterns runs against every dataset; a subject absent from
 // a given exam year never matches and stays NULL. Matching is unanchored
-// first-match, like Rust's Regex::captures.
+// first-match.
 func ParseScores(diemThi string) map[string]float64 {
 	out := make(map[string]float64)
 	if diemThi == "" {
@@ -161,8 +153,8 @@ func ParseScores(diemThi string) map[string]float64 {
 		if err != nil {
 			continue
 		}
-		// Unreachable given the pattern shape, but kept for parity with
-		// transform.rs:136 (is_finite).
+		// Unreachable given the pattern shape; kept so a widened pattern can
+		// never write Inf or NaN into a score column.
 		if math.IsInf(v, 0) || math.IsNaN(v) {
 			continue
 		}
@@ -172,21 +164,20 @@ func ParseScores(diemThi string) map[string]float64 {
 }
 
 // ErrNoColumns is returned when the fixed-column path is used on a config that
-// has no columns: mapping. Rust panics here via .expect() (transform.rs:162);
-// returning an error is the Go-idiomatic equivalent and is unreachable in
-// practice, since only the non-2016 path calls this.
+// has no columns: mapping. Unreachable in practice, since only the non-2016 path
+// calls TransformRow.
 var ErrNoColumns = errors.New("transform: config has no columns mapping")
 
 // TransformRow extracts one row into a ParsedRow using fixed column indices,
-// the 2017-family path. 2016 uses runtime format detection instead.
+// the 2017 path. 2016 uses runtime format detection instead.
 func TransformRow(raw []reader.Cell, cfg *config.DatasetConfig) (*ParsedRow, error) {
 	cols := cfg.Columns
 	if cols == nil {
 		return nil, ErrNoColumns
 	}
 
-	// Trimmed accessor, mirroring the closure at transform.rs:163-167. Out-of-range
-	// indices yield "" rather than an error, matching unwrap_or_default().
+	// Trimmed accessor. Out-of-range indices yield "" rather than an error, so a
+	// short row produces empty fields instead of failing.
 	get := func(idx int) string {
 		if idx < 0 || idx >= len(raw) {
 			return ""
@@ -198,9 +189,9 @@ func TransformRow(raw []reader.Cell, cfg *config.DatasetConfig) (*ParsedRow, err
 	ngaySinh := get(cols.NgaySinh)
 	soBaoDanh := get(cols.SoBaoDanh)
 
-	// diem_thi is read WITHOUT trimming (transform.rs:172-175), unlike the three
-	// fields above. Harmless because the score patterns are unanchored, but it is
-	// the shipped behaviour — do not "tidy" it.
+	// diem_thi is read WITHOUT trimming, unlike the three fields above. Harmless
+	// because the score patterns are unanchored, but it is the shipped behaviour
+	// — do not "tidy" it.
 	diemThi := ""
 	if cols.DiemThi >= 0 && cols.DiemThi < len(raw) {
 		diemThi = raw[cols.DiemThi].Str