TASK-025: validation fixes — per-method tests, assert(slot), housekeeping

Address validation findings from /groundwork:validate:
- Added 7 per-method dispatch tests, empty-handler guard test, and
  single-resource non-root path conflict test in webserver_on_methods_test.
- lambda_resource.hpp: replaced defensive null-slot fallback with
  assert(slot) + invariant comment; added <cassert>.
- webserver.cpp: block comment on the seven on_* forwarders.
- Marked TASK-025 Done in task spec and _index.md.
- Recorded unworked review issues (2 major, 38 minor) for follow-up.

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

Author: etr

specs/tasks/M4-handlers/TASK-025.md

@@ -28,4 +28,4 @@ Add the lambda-first handler model that lets a stateless endpoint be registered
 **Related Requirements:** PRD-HDL-REQ-001, PRD-HDL-REQ-002
 **Related Decisions:** DR-004, §4.7
 
-**Status:** In Progress
+**Status:** Done

specs/tasks/_index.md

@@ -107,7 +107,7 @@ Nominally: **13 sequential tasks**, each S–XL. Most other tasks parallelize of
 | TASK-022 | Snake_case `render_*` overrides on `http_resource` | M4 | Done | TASK-021 |
 | TASK-023 | Smart-pointer `register_resource` overloads | M4 | Done | TASK-014 |
 | TASK-024 | `register_path` and `register_prefix` (replace `bool family`) | M4 | Done | TASK-023 |
-| TASK-025 | Lambda handler entry points `on_*` | M4 | Not Started | TASK-005, TASK-009, TASK-014 |
+| TASK-025 | Lambda handler entry points `on_*` | M4 | Done | TASK-005, TASK-009, TASK-014 |
 | TASK-026 | Generic `webserver::route(method, path, handler)` | M4 | Not Started | TASK-005, TASK-025 |
 | TASK-027 | 3-tier route table with LRU cache | M5 | Not Started | TASK-005, TASK-014, TASK-021, TASK-024, TASK-025, TASK-026 |
 | TASK-028 | Routing-semantics regression gate | M5 | Not Started | TASK-027 |