perf: index once in rewalk, bind.refresh and digest.laws_of (#228)

[ngngardner]

Fixes three of the four per-item scans in #228. No LAWS.bend changed, and bolt's lint output is byte-identical.

Changes

• rewalk: each site's result is now checked once per straight piece. Before, it was re-checked for every twin a site was compared against. other_go, earlier and report now compare the stored flags, and other_full, earlier_one and earlier_nar are gone. The proofs of rewalk_local_counts in src/rules/PROOF.bend now go through Rewalk.rate.
  • Stress test, 150 identical +aN = walk(xs) lets: 14.7 s → 0.14 s.
  • Trade-off: a site with no twin now costs one walk of the piece, where before it cost none. A file of 400 lets with no twins went from 0.24 s to 0.68 s. Linting bolt with only rewalk on shows no measurable difference.
• bind.refresh: one pass over the annotated binders with a cursor, instead of a full binder search for each environment entry. The binders are in source order, which Bound promises. A binder earlier than the last one read restarts the cursor, so the result stays right whatever order the environment is in. Old and new gave the same result on all 20,461 scopes of 102 files in the tree.
• digest.laws_of: the outline items are read with a cursor that moves forward from law to law, instead of being searched from the start for each law. The Law lists were byte-identical on all 8 LAWS.bend files and on a synthetic file with 3,000 laws.
• Both cursors, seek and from_line, carry the stop test as a Bool instead of wrapping the recursive step in a thunk, as AGENTS.md asks. thunk (U015) reports none of the new code.

Not changed: unused

unused_counts states the foreign-line lookup as List.contains over any binds. A faster exact lookup, such as a trie or a sorted cursor, would need a lemma that U32.is_eq(a, b) means a == b, and Base has none. That list is also empty in any file without foreign defs.

With scan turned on, the refresh, laws_of and rewalk per-site findings are gone. rewalk still has two scan findings: one walk per site, and the pairwise twin comparison that the law states.

Gate

• bend PROOF.bend prints ALL PROOFS CHECK in all 7 directories.
• bend main.bend -o bin/bolt.bin builds.
• bolt --gpu off gives 0 errors, 73 warnings, the baseline.

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https://claude.ai/code/session_01X7i62UAece6mwMRV3Y1b9B

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