TASK-028: validation fixes — cache-tier pins, miss baseline, _index Done

Address validation findings from /groundwork:validate (2 passes):
- test/unit/routing_regression_test.cpp: fix always-green assertion in
  the overlapping-routes test (was *hp == first || *hp == second);
  add unregistered_path_yields_miss as the pure-miss baseline; add
  second_lookup_same_path_hits_cache_tier and
  unregister_invalidates_cache_entry to pin the LRU cache behaviour
  the gate's own rationale calls out; add FIXME(TASK-036-prereq)
  comment on the non_numeric assertions documenting the v1/v2 custom-
  regex divergence.
- specs/tasks/_index.md: normalise TASK-028 status from Not Started to
  Done — the row was stale before this branch.
- Recorded unworked review issues (31 minor findings: REGRESSION.md
  prose updates, naming/style nits, doc tweaks).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: etr

specs/tasks/_index.md

@@ -110,7 +110,7 @@ Nominally: **13 sequential tasks**, each S–XL. Most other tasks parallelize of
 | TASK-025 | Lambda handler entry points `on_*` | M4 | Done | TASK-005, TASK-009, TASK-014 |
 | TASK-026 | Generic `webserver::route(method, path, handler)` | M4 | Done | TASK-005, TASK-025 |
 | TASK-027 | 3-tier route table with LRU cache | M5 | Done | TASK-005, TASK-014, TASK-021, TASK-024, TASK-025, TASK-026 |
-| TASK-028 | Routing-semantics regression gate | M5 | Not Started | TASK-027 |
+| TASK-028 | Routing-semantics regression gate | M5 | Done | TASK-027 |
 | TASK-029 | Naming consistency — `stop_and_wait`, `block_ip`/`unblock_ip` | M5 | Not Started | TASK-014 |
 | TASK-030 | `_handler` suffix renames + `explicit` constructor | M5 | Not Started | TASK-014 |
 | TASK-031 | Handler error-propagation contract (DR-009) | M5 | Not Started | TASK-027, TASK-030 |