feat(collector): prepare paired proposer signature collection

[shane-moore]

Merge order: #1311 → #1312 → #1313. All three target epbs; rebase remaining branches after each merge.

Problem, Evidence, and Context (Required)

Second of three PRs for #1309, dependent on #1311. SIP-94 requires one proposer packet containing block and envelope shares, while block completion must not wait for envelope quorum. Two independent collection calls would emit two packets.

Change Overview (Required)

• Add paired proposer collection and a wait-only path for an already registered root.
• Preserve bounded same-packet block-companion evidence for envelope shares, including packets received before registration.
• Reconstruct block and envelope signatures independently within the existing collector lifecycle. Existing duty callers keep their current behavior until the final layer activates the fold.

Review only this layer.

Risks, Trade-offs, and Mitigations (Required)

The shared collector is the main regression risk. Companion policy is opt-in and immutable after registration; evidence is bounded, signature-specific, and removed with existing slot cleanup. This avoids a separate packet inbox or another lifetime-managed cache.

Validation (Required)

Executed independently with the first two layers applied:

• cargo check --workspace --tests --locked: passed.
• cargo nextest run --locked -p signature_collector -p anchor_validator_store: 232 passed, 0 skipped.
• make lint, make cargo-fmt-check, git diff --check: passed.
• Coverage includes early arrivals, both entry orders, independent thresholds, companion mismatches, immutable registration, cached results and expiry without respawning a collector.

Rollback (Required for behavior or runtime changes; optional otherwise)

Before activation, revert this preparatory layer. After activation, revert the final layer first.

Blockers / Dependencies (Optional)

Merge #1311 first. The source branch is stacked in shane-moore/anchor. This PR targets epbs because its predecessor branch exists only in the fork; the upstream diff includes the prerequisite until it lands.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 74.20382% with 81 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (epbs@58e2255). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
anchor/signature_collector/src/lib.rs	81.78%	45 Missing ⚠️
anchor/validator_store/src/testing/common.rs	33.96%	35 Missing ⚠️
anchor/message_receiver/src/manager.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             epbs    #1312   +/-   ##
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  Coverage        ?   80.93%           
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  Files           ?      181           
  Lines           ?    42337           
  Branches        ?        0           
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  Hits            ?    34265           
  Misses          ?     8072           
  Partials        ?        0           

Flag	Coverage Δ
rust	80.93% <74.20%> (?)

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[shane-moore]

Reviewed 8d9a52cc. No findings.

The paired API emits one proposer packet and waits only for the block signature. Envelope reconstruction is independent, same-packet companion evidence remains bound to the signature bytes, and the wait-only path cannot recreate an expired collector. Existing callers retain unrestricted collection.

Verified hosted debug and release logs: 1,030 tests passed in each on the same source tree as this head, including all ten new proposer tests and the existing collector fallback tests. Local-testnet CI also passed.

This is collector preparation only: the paired API has no production caller yet, and two-entry proposer admission remains for #1313.

Reviewed by gpt-6 astra high.