[AZ-500] Cycle 4 Step 17: retrospective + close cycle

Adds retro_2026-05-12_cycle4.md, structure_2026-05-12_cycle4.md, and
the deploy_cycle4.md report that was dropped from the Steps 12-15
sync commit. Appends 3 new lessons to LESSONS.md (12/15 ring buffer)
on transitive major-version bumps, exposed pre-existing bugs, and
single-task-cycle metric framing. State advances to cycle 5 / step 9
(awaiting next New Task invocation).

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/LESSONS.md

@@ -37,6 +37,12 @@ If the enum's wire string happens to match a member name case-insensitively (e.g
 
 ## Ring buffer (last 15 entries — newest at top)
 
+- [2026-05-12] [dependencies] Major-version bumps of direct deps cascade through transitives; the task spec must list the transitive packages whose major version changes as a result OR explicitly note "transitive major-version drift not analyzed in spec" — verify with `dotnet restore --dry-run` against a scratch branch before writing the spec (cycle 4: AZ-500 surprise-bumped `Microsoft.OpenApi` 1.x → 2.x via the `Microsoft.AspNetCore.OpenApi` 8.0.25 → 10.0.7 path; forced an unscheduled Swashbuckle bump + Program.cs refactor mid-implementation).
+  Source: _docs/06_metrics/retro_2026-05-12_cycle4.md
+- [2026-05-12] [process] When a scope-protected task newly *exposes* a pre-existing bug elsewhere in the codebase (vs. introducing a new one), surface it as a recommended follow-up PBI in the batch report AND list it as a "newly exposed bug" separate from "newly introduced findings" in the deploy report — bugs that already existed don't count as cycle-introduced regressions, but they must not be silently re-buried (cycle 4: AZ-500's bootstrap fix unmasked the pre-existing `scripts/run-performance-tests.sh:417` `grep -o | wc -l` + `pipefail` bug).
+  Source: _docs/06_metrics/retro_2026-05-12_cycle4.md
+- [2026-05-12] [process] When a cycle has a single non-functional task (migration / refactor / dependency hygiene), the retro must reframe the metric set around continuity (0 regressions), forward-resolution (prior findings closed by the bump itself), and unblocking (capabilities now exercisable end-to-end) — task count + complexity points read as misleading flatlines that look like under-productivity (cycle 4: AZ-500 alone delivered 5 SP vs cycle 3's 18 SP, but the cycle's value was forward-resolving 2 cycle-3 advisories and finally executing PT-01..PT-08 end-to-end against the migrated build).
+  Source: _docs/06_metrics/retro_2026-05-12_cycle4.md
 - [2026-05-12] [process] For cross-team blockers (admin team must supply config values, etc.), prefer an Option-B forcing function (ship the validation/scaffolding with prod-empty config that fails-fast at deploy) over deferring the entire task — the fail-fast contract makes the cross-team conversation impossible to skip and ships the in-workspace work in the current cycle (cycle 3: AZ-494 shipped iss/aud validation with empty prod appsettings so deploy must supply real values).
   Source: _docs/06_metrics/retro_2026-05-12_cycle3.md
 - [2026-05-12] [process] ACs that prescribe a specific measurement or sentinel mechanism (e.g. "per-item latency < 50ms", "guard fires when DB name contains _test") should also prescribe — or explicitly defer — the path for collecting / enforcing it, or implementations will substitute proxies / equivalents that look like spec drift in review (cycle 3: AZ-492 PT-08 per-item gate cost became a derived proxy; AZ-493 DB-name guard became Host-allowlist).