docs(buck2): resolve second-context reuse delta

[schickling-assistant]

Problem

The S8 second-context sample reported 633 local actions and recorded DELTA-001 as a possible action-key stability defect. That sample was the first build at its rebased revision, so it did not establish that the shared cache was warm for that exact revision.

Goal

Determine whether the same successful actions reuse across a normal context and the isolated second context at one pinned revision, then close DELTA-001 if the action keys are stable.

Decisions

• Warm the exact revision in the normal context before running the isolated context.
• Treat the remaining failing typecheck separately from key stability. A failed action cannot provide a successful cache result.

Verification

At revision 948d397a2e:

Context	Cache queries	Cached	Local
Normal context	1,195	648	547
Isolated second context	1,195	1,192	1

The remaining action failed identically in both contexts. Both cache queries used action digest 76ecc7d96ba19cabf193f3e0fc6f48509e214e0b4b1560f75c5dca70c2cf5297:142, and the command arrays were byte-for-byte equal. The experiment therefore resolves DELTA-001 as a test-ordering artifact.

• axe vrs check --profile strict context/buck2/04-reuse: pass
• git diff --check: pass
• Full strict validation reports only the existing decision-shape errors in decisions 0035 and 0036.

The BUCK-R16 second-context evidence is context/buck2/.experiments/2026-09-19-second-context-key-stability.md.

Complexity

No new implementation complexity. Build-machinery ledger net: 0 lines; this PR changes evidence documents only.

Concerns

The sampled branch remains red for a separate reason: the packages/@overeng/megarepo preferSchemaOverJson warning makes tsgo exit 2 during //:quick.

Friction & bottlenecks

The shared heavy-command gate serialized the two proof builds as intended. No new recurring friction was found in this amendment.

Follow-ups

• The landing seat for feat(buck2): close JavaScript and TypeScript authority migration #1283 and feat(buck2): add standalone root and aggregate gates #1301 owns the preferSchemaOverJson warning. This PR does not fix it.

References

• Stacked on feat(buck2): add standalone root and aggregate gates #1301
• Resolves DELTA-001 as a test-ordering artifact

Posted on behalf of @schickling

field	value
agent_identity	dev3.direct.omp.4uu54jth
session	unknown
agent_persona	generalist
agent_supervisor	unavailable
agent_tool	OMP
agent_tool_version	18.2.6
agent_runtime	OMP 18.2.6
agent_model	openai-codex/gpt-5.6-sol
worktree	2026-09-19-key-stability/2026-09-19-key-stability
tooling_profile	dotfiles@965ef7e-dirty

[schickling-assistant]

Superseded: this record landed on main via #1319 (experiments) / #1301 (key-stability evidence, DELTA-001 removal). Closing per the 2026-09-22 stack handoff.