docs(vcr): record optimizer_bridge/selector frontier second arc + #509 negative result (#242)

[avrabe]

Behavior-frozen VCR-ORACLE-001 (#242) traceability update — a docs/measurement artifact, no code touched.

Appends a dated slice to VCR-ORACLE-001's history covering the 2026-06-26 second arc:

• Optimized path silently miscompiles register-exhausting i32 folds (no spill, no decline) #496 (PR fix(optimizer): optimized path declines to direct on R4-R8 exhaustion (#496, #242) #502) — optimized-path R12-exhaustion → decline-to-direct; oracle r12_spill_496_differential.py.
• arm: functions needing the AAPCS stack-arg path (>8 scalar params, or any 64-bit stack param) are skipped, not lowered #503 (PR fix(arm): lower functions needing the AAPCS stack-arg path with >8 scalar params/args (#503, #242) #504, shipped v0.16.0) — AAPCS stack-arg >8-cap lift on the shipped direct selector; oracle stack_args_503_differential.py. First shipped-path member of the cluster (hence the v0.16.0 minor; the optimized-path-only Optimized path: small leaf functions clobber callee-saved r4-r8 without a saving prologue (latent ABI bug) #490/Optimized (non-relocatable) path miscompiles block/br_if: branch target lands mid-instruction #483/Optimized path silently miscompiles register-exhausting i32 folds (no spill, no decline) #496/Optimized path silently miscompiles br_table — selector elided, all arms fall through (#483/#500 family) #507 ride along).
• Optimized path silently miscompiles br_table — selector elided, all arms fall through (#483/#500 family) #507 (PR fix(arm): optimized path declines br_table to the direct selector (#507, #242) #508) — optimized-path br_table dispatch-drop → decline-to-direct; oracle br_table_507_differential.py.
• Direct selector miscompiles value-returning br_table — carried value dropped (affects --relocatable/shipped path) #509 — load-bearing NEGATIVE RESULT: the direct selector (shipped --relocatable path) drops the carried value of a value-returning br/br_if/br_table-direct. The Optimized path silently miscompiles br_table — selector elided, all arms fall through (#483/#500 family) #507 decline-to-direct shortcut cannot fix it — the IR carries no block-result arity (WasmOp::Block is a unit variant), so a non-empty-vstack decline can't tell a carried result from an unwound void-target value and over-refuses valid code. Recorded so it isn't re-attempted. This is the sharpest evidence yet for VCR-SEL-001's selector-collapse motivation; the real fix is arity-threading (cross-cutting, silicon-gated, not an idle-tick increment).

Verification

• Docs-only, behavior-frozen by construction.
• Frozen byte gate bit-identical (2/2).
• rivet validate non-xref ERROR count 0 (unchanged 49 errors / 75 warnings / 0 broken xref baseline).

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