[proggeramlug]

Startup-snapshot census, instrumented builds only (cfg(perry_gc_instruments)), off by default. Follows #13935 toward the execution-prefix snapshot model: run init in its real order at build time up to the first input that cannot be guarded, image the heap there, and guard the inputs read before that point at startup.

What this adds

• An ordered startup input record (env, argv, locale, tty, ids, files, clock, entropy) with values and instruction positions, streamed as it happens. A per-thread depth guard keeps the recorder from recording its own allocator's clock reads, and the clock/env hooks use fixed buffers.
• Cut computation under R1/R2 (frames still open at the cut must be resumable), with replay counterexamples.
• Census page policy fix: after first-write tracking ends, a rule-4 conflict could re-protect every tracked heap page while the policy said writable; the handler then did not take the next access. Bulk protection now follows the policy.

Gates

• Six startup witnesses (hello, commander_help, commander, fastify_inject, richards, binary_trees): .text identical, callgrind Ir delta 0, proven-site identities gained/lost 0.
• The 34 census fixtures of runtime: build-time init snapshot census (instr-only): Supabase check differ=0, D 1,643 (93.9%), production bytes = main #13935 still pass with their off arms red; recorder, GC, abort and page-boundary fixtures pass in both watch modes (the page-boundary fixture stops with exit 139 on the previous head).

Supabase census (one run)

2,895 inits, 8,882.5 M init Ir. Cuts by the semantic input classes:

cut	guards	stops before	% of init Ir
P1	env, argv, locale	init 1230 (tty read)	61.4%
P2	+ tty	init 1488 (uid/gid read)	63.6%
P3	+ ids	init 2455 (directory read)	88.6%

undici's init (1652) falls inside P3. Draft: native reads that the semantic classes do not cover yet (exe path, /proc/self/maps, CPU affinity, cgroup cpu.max, clock reads) are being classified next; until then the cuts are upper bounds.

[coderabbitai]

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