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/sqlitedb/sqlitedb.go

@@ -5,17 +5,16 @@
 // cgo-free — grate, excelize and yaml.v3 are pure Go too, so CI can compile with
 // CGO_ENABLED=0 and no C toolchain.
 //
-// It is a machine-transpiled SQLite rather than the upstream C amalgamation that
-// Rust's rusqlite --bundled vendors (libsqlite3-sys 0.30.1). Two things make that
-// acceptable: this parser uses only plain SQL — no CTEs, window functions,
-// triggers or extensions — and the differential gate compares a full-table
-// SHA-256 plus PRAGMA table_info/index_list against live Rust output.
+// It is a machine-transpiled SQLite, not the upstream C amalgamation, which is
+// acceptable only because the parser sticks to plain SQL: no CTEs, window
+// functions, triggers or extensions. Adding any of those means re-verifying the
+// driver.
 //
 // Verified on linux/arm64 with v1.56.0 (SQLite 3.53.3): the full DDL including
 // the partial idx_ten_cum_thi index, INSERT OR REPLACE, and VACUUM.
 //
-// Fallback if the differential gate ever implicates the driver: mattn/go-sqlite3
-// is upstream C at the cost of cgo.
+// Fallback if the driver is ever implicated: mattn/go-sqlite3 is upstream C at
+// the cost of cgo.
 package sqlitedb
 
 // Registers "sqlite" with database/sql.