build: per-variant objdirs — variants coexist, rebuilds go incremental (#740)

[InauguralPhysicist]

Closes #740

What

Every runtime variant was a single whole-program gcc invocation writing src/eigenscript — an ASan build destroyed the release binary, every edit cost a full 22-TU rebuild, and the project was absorbing the collision through the #681 fingerprint guard plus a standing CLAUDE.md prohibition.

Now each of the 11 variants compiles into its own build/<variant>/ objdir with -MMD/-MP header-dependency tracking and links build/<variant>/eigenscript; the phony target re-points src/eigenscript at it, so every existing consumer of that path — 35 test/tool scripts, CI, install, consumers — works unchanged. Target names, per-variant flags, and output messages are byte-identical.

Measured:

• make asan → make: ~90 s full rebuild → 0.2 s relink
• touch vm.c → 1 TU recompiles; touch vm.h → exactly its 12 includers
• all 11 variant objdirs coexist under build/

The hard-link subtlety (a symlink prototype failed 58 suite checks)

The alias is a hard link, not a symlink. The runtime resolves the stdlib relative to /proc/self/exe, which dereferences a symlink into build/<variant>/ — and lib/ stops being a sibling. The symlink prototype failed every executable-relative-stdlib test (bundle, replay, import). The hard link keeps the exec'd path at src/eigenscript, and coexistence is preserved because each variant's objdir file keeps its own name.

Consequences handled:

• pgo/coverage (whole-program by nature) and build.sh rm -f the alias before writing, so they never truncate the shared inode under a variant's binary.
• A real guard trip during verification exposed that the guard's stat didn't dereference — its metadata half described the link while its cksum half read the target. It now uses stat -L, and [99d]'s restore re-creates a symlink alias instead of materializing a copy. (Both are no-ops for the hard link; they make the guard correct under any alias form.)

What's retired vs kept

• Retired: the cross-variant half of the "never rebuild one variant while a suite runs against another" prohibition — variants no longer share a binary.
• Kept: the Test runner should fail loud when the binary is rebuilt mid-run, not silently grade against it #681 fingerprint guard and its [99d] self-test. Re-pointing the alias (make <other-variant>) or relinking the same variant mid-suite is still a mid-run swap, and the guard is what catches it.

Gate

• release suite 3364/3364; ASan/UBSan detect_leaks=1 3368/3368, leak tally 0
• all 10 locally-buildable variants built through the engine; make -n full verified (libpq absent locally — the db extension CI leg is the real build)
• build.sh interop verified: writes its own file, variant binaries untouched, stdlib resolves

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR reworks the build system so each runtime variant builds incrementally into its own build/<variant>/ object directory, while keeping the historical src/eigenscript path working via a retargeted hard-link alias. It also updates the test runner’s binary-fingerprint guard and supporting docs/scripts to account for the new alias behavior.

Changes:

• Introduces a per-variant objdir engine in Makefile (pattern rules + -MMD/-MP deps) and retargets src/eigenscript to the last-built variant via hard link.
• Updates tests/run_all_tests.sh fingerprint guard to use stat -L (symlink-safe) and adjusts the [99d] self-test restore logic.
• Updates supporting documentation (CLAUDE.md, .claude/rules/..., CHANGELOG.md) and build.sh to avoid truncating a hard-linked alias.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/run_all_tests.sh	Makes the binary fingerprint guard symlink-aware and updates the [99d] self-test logic for the new alias behavior.
Makefile	Implements per-variant build/<variant>/ objdirs with dependency tracking and hard-link retargeting for src/eigenscript.
CLAUDE.md	Updates the build guidance to reflect per-variant builds and the src/eigenscript hard-link alias.
CHANGELOG.md	Documents the per-variant objdir change and its operational implications.
build.sh	Removes src/eigenscript before compiling to avoid writing through a hard link.
.claude/rules/c-runtime-memory.md	Updates project guidance about variant coexistence and mid-suite alias swaps.

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