fix: harden actionable error-signature reads

[jmagar]

Summary

Follow-up to the merged systematic audit in #196. The adversarial post-merge review found two remaining items, tracked as AUD-035 and AUD-036 in reopened master issue #194.

AUD-035 — remove dead returned materialization

• Remove production-dead SignatureRow.normalizer_version from the returned DB row model.
• Stop selecting/decoding that unused value in unaddressed-error paging and exact signature lookup.
• Keep normalizer_version load-bearing in primary-key predicates, joins, grouping, writes, and lookup arguments.
• Remove the obsolete #[allow(dead_code)] from the test-only CortexService::with_os_adapter constructor, which has real test consumers.

AUD-036 — keep the actionable list actually actionable

error_signatures is durable across normalizer versions, but the public list/ack/unack API identifies a signature by hash only and mutations target the active NORMALIZER_VERSION. Previously, a historical/non-current row could therefore appear in unaddressed_errors even though the current API could not acknowledge it.

• Scope read_unaddressed_page and its recent-window aggregate to the active normalizer version.
• Preserve historical rows in SQLite for audit/history.
• Add DB and service regressions proving non-current rows stay durable but never surface in the actionable list.

This branch is based on current main after #196 and Agent Observatory PR #195.

Verification on current head 8b4346a2

• cargo fmt --all + git diff --check
• cargo nextest run unaddressed_errors error_signatures -j 1: 11/11 passed
• new API regression: unaddressed_errors_hides_non_current_normalizer_versions passes
• DB version-filter regression passes
• cargo clippy --all-targets --all-features --locked -- -D warnings
• repository pre-push router: version sync, module-size, strict Clippy all green

Final exact-head verification is complete.

Fixes #194

Final exact-head evidence

• Durable full nextest: 2953/2953 passed, 2 skipped, exit 0 (852.613s; slow labels were passing load classifications only).
• cargo clippy --all-targets --all-features --locked -- -D warnings: passed.
• cargo test --doc: passed warning-free.
• cargo machete: no unused dependencies.
• cargo deny check: advisories/bans/licenses/sources all green.
• cargo audit --ignore RUSTSEC-2023-0071: no additional vulnerabilities.
• Version sync: all 16 version-bearing files at 3.13.2.
• Agent Observatory contracts, transcript env validators, plugin validation, formatting, just check, module-size, whitespace and clean-tree checks: passed.
• GitHub CI is fully green, including CI Gate, MCP Integration, Tests, Clippy, cargo-deny, Formatting, Repository Contract, CodeQL, version sync, and all analysis lanes.
• Adversarial PR review: zero unresolved review threads; no further source findings after AUD-036.

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