codegen: cwd hash baked into __perry_cap_* symbols defeats reproducible builds and every cross-directory A/B (#7038/#7131/#7172 family)

[proggeramlug]

Found while building PR #7176's byte-neutrality harness: Perry bakes a hash of the current working directory into __perry_cap_* symbol names, so the same source compiled from two different directories produces different IR/objects.

Measured: with per-run mkdtemp working directories, 29 of 29 same-compiler control runs failed (same compiler, same source, different cwd → different symbols). The harness had to pin a fixed working directory to make any comparison valid.

Why it matters

• Reproducible builds are impossible across checkouts/machines — two developers building identical source get different binaries by construction.
• Every IR/object A/B harness must know to pin cwd, or it chases phantoms — three separate measurement compaigns have now built cwd/normalization workarounds independently (perf(repsel): make the Ptr<Shape> alloc-site buckets mean what schedulers read them as (#7170 R0) #7176's harness, perf(repsel): stop charging Perry's own cjs_wrap preamble to the Ptr<Shape> report (#7152) #7171's IR comparison, the census determinism gate).
• Same family as the fixed determinism bugs: codegen: same input compiles to different LLVM IR run-to-run — HashMap iteration order in closure-source registration breaks reproducible builds #7038/fix(codegen): sort closure-source emission so the same input compiles to the same IR (#7038) #7039 (closure-source order), codegen: object emission is nondeterministic on Linux — the LLVM temp module name embeds pid + nanotime #7131/fix(codegen): content-address LLVM temp IR names (#7131) #7135 (temp names), codegen: perry_method_<Class>__<name> emission order is HashMap-nondeterministic when a method name is shared across classes (#7038/#7039 family) #7172 (method emission order, open). The cwd hash is the largest remaining member.

Fix shape

Key __perry_cap_* on something checkout-invariant — e.g. the module's repo-relative path or a content hash of the source — rather than the absolute cwd. (If the cwd hash exists to disambiguate concurrent builds of different projects, a project-root-relative identity preserves that without breaking reproducibility.)

Acceptance

Same source, same compiler, two different working directories → byte-identical objects (the exact control #7176's harness runs). Red today on all 29 census workloads probed.