Port Bun stack formatting and error diagnostics

Tracks the focused port of js/bun/test/stack.test.ts from the vendored Bun corpus.

Scope:

• normalize JavaScriptCore stack frames to Bun-compatible at function (file:line:column) output
• preserve function-name updates, async frames, and anonymous frames
• cover Bun.inspect(Error) through the same stack representation
• port fixture line/column behavior and subprocess error diagnostics
• add an exact-corpus regression and keep the full home_test gate green

Parent tracker: #66

Implemented and pushed to main in c0756a8.

Completed:

• activated the byte-identical js/bun/test/stack.test.ts corpus (plus byte-identical custom/system/line-column fixtures)
• normalized JavaScriptCore frames into Bun-compatible at function (file:line:column) stacks
• preserved dynamic function-name overrides, async frames, anonymous boundaries, and shared Bun.inspect(Error) behavior
• routed both diagnostic fixtures through real home-debug run subprocesses while preserving exit status and canonicalizing Bun-visible getter/system-error formatting
• ported Bun's stack-location behavior into normalizeBunSnapshot
• added a permanent bootstrap regression asserting 6 passes and the single upstream todo

Validation on the final rebased tip:

• exact corpus: 6 passed, 1 todo, 0 failed, 0 unsupported
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: passed
• Zig formatting and git diff --check: passed

Parent tracker: #66