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/testdata/reader-fidelity-hashes.tsv

@@ -1,13 +1,12 @@
-# Canonical cell-dump SHA-256 per input file, produced by the Rust/calamine
-# ground truth (parser/examples/dump_cells.rs). The Go reader must reproduce
-# each hash exactly. Hashes only - the dumps themselves are real student PII
-# and are never committed, per parser/tests/fixtures/README.md.
-# FROZEN. These hashes were produced by the Rust/calamine parser, which has
-# since been removed, so they cannot be regenerated. They remain a real
-# regression guard: any change to the Go reader that alters a single cell of any
-# input file listed below fails the suite. If the inputs themselves ever change,
-# the affected rows must be re-derived deliberately and reviewed, not refreshed
-# in bulk.
+# Canonical cell-dump SHA-256 per input file. The reader must reproduce each
+# hash exactly. Hashes only - the dumps themselves are real student PII and are
+# never committed. parser/cmd/dumpcells prints the stream a hash is taken over,
+# which is how a failure is narrowed from a file to a cell.
+# FROZEN. These cannot be regenerated, so a mismatch is a reader bug until
+# proven otherwise, never a cue to refresh this file. Any change to the reader
+# that alters a single cell of any input listed below fails the suite. If the
+# inputs themselves ever change, re-derive only the affected rows, deliberately
+# and reviewed, not in bulk.
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