ParseTable carrier: prepare grammar once above the assemble fold (validate-once)

[gunbai-bot]

ParseTable carrier — prepare the grammar once, not per module

Finding

grammar_validate_for_parse runs five whole-grammar checks (undefined-nonterminals, duplicate-productions, left-recursion, choice-ambiguity, nullable-ambiguity) plus the FIRST/nullable analysis — all pure functions of the invariant 841-row grammar — inside parse_module. So a door call over a K-module import closure re-validated the same grammar K times, and compute_grammar_first_analysis ran twice per module (once for the ambiguity check, once in parse_table_for_production).

The ParseTableRealization carrier already had grammar_digest / nullable_set / first_rows / materialization slots — but the cache lookup is Miss-stubbed (resolve_cache_miss), so nothing was reused. This was a recompute-per-module bug, not a missing cache.

Measured (release claim_batch, wet, std.determinism): one parse+normalize ≈ 14.7s, of which grammar_validate_for_parse alone ≈ 9.2s, re-paid every module.

Fix (correctness by construction)

A PreparedGrammar carrier + prepare_grammar (well-formed check + validate + analyze, once) hoisted above fold_list in program_assembly_fold_ingest; parse_module_prepared reuses it for every module. A door call now validates its closure once, not K times — structurally, no timing gate.

• parse_module / parse_production / parse_table_for_production / program_assembly_read_to_normalized_root keep backward-compatible wrappers (prepare ∘ parse). Only the internal fold_step signature changed (no external callers).
• Empty-ingest is guarded so zero modules never pay the one-time preparation.

Proven by execution

• Behavioral equivalence — the ledger histogram witness stays green with identical causes (determinism→fact_density_hollow_alias_locus, error_primitives→resolve_reason_unbound_symbol, bit→resolve_ambiguous_export).
• Amortization — parse_table_grammar_memo_multi_file_ingest_parses green; a 2-file fold costs ~9.2s = one preparation (not ~18.4s = two).
• New empty-ingest guard witness green in 4ms (no prepare, no parse).

Follow-up (not this PR)

Cross-call content-addressed memo (sharing the prepared grammar across door calls) is gated on wiring the currently Miss-stubbed cache backend. Draft until by-execution sign — load-bearing parse stage.

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