Comment diagnostics to de-facto standard: unterminated-comment error (Pascal) + -Wcomment (C)

• Type: feature (lexer diagnostics). Track A (shared lexer.inc) + Track C (C frontend comment scan). Owner: a-agent (2026-07-08).
• Origin: investigation of nested-comment vs literal lexing (see [[tooling-nested-comment-brace-selfhost-guard]]). Guiding principle: be compliant with de-facto standards.

Investigation result (already compliant, except diagnostics)

Verified empirically against pascal26 (both frontends, both roles):

• Literals shield comment delimiters and vice-versa — universal, correct. ✅
• Pascal { } nests, {$mode delphi} off = FPC 3.2.2. ✅
• C /* */ does NOT nest = C standard / gcc / clang. ✅
• Mixed-delimiter inertness ({ (* *) }, (* { } *)) works. ✅

The behaviour is the de-facto standard; no lexing-semantics change. Two DIAGNOSTIC gaps vs the standard remain:

1. Pascal: unterminated comment gives a garbage cascade (PRIMARY)

Every compiler (FPC/gcc/clang) reports unterminated comment at the OPENING location. pascal26 instead cascades:

{ this comment never closes ...        (opened line 4)
pascal26:28: error: undefined variable (interface)   <- bogus, far away

This is the exact original pain (a lone brace in an .inc comment crashed self-host as unexpected character at a past-EOF line). Fix: when a { } or (* *) comment scan hits EOF unclosed, emit unterminated comment (opened at line X, col Y). Makes the external lint ([[tooling-nested-comment-brace-selfhost-guard]]) redundant for the fatal case.

2. C: no -Wcomment (SECONDARY)

gcc -Wall / clang: warning: "/*" within comment [-Wcomment] when a /* appears inside a block comment (a likely mistake — C comments do not nest). pascal26's C frontend is silent. Add the warning to match.

Note: a per-{ "ignoring delimiter" warning for the PASCAL nesting case is NOT standardized — FPC nests intentionally, so it would fire on every legitimate nested/balanced comment (688 in-tree). Not added.

Gate

• Track A: make test + self-host byte-identical.
• Track C: C tests green + conformance no-regress.
• New: a Pascal source with an unterminated {/(* comment reports unterminated comment at the open; a C /* /* */ compiles AND warns -Wcomment; well-formed comments unaffected.

Log

• 2026-07-08 — resolved, commit 0aa9505d.