config: multi-error YAML validator + --validate-config CLI flag

The existing loader throws ConfigError on the first problem it hits.
A customer with a tool-specific equipment.yaml that has six issues
sees one, fixes, restarts, sees the next, fixes, restarts — six
edit-restart cycles before the server even binds.  Day-1 friction
is the top support ticket source in fab integrations.

This commit adds a parallel validator that does a separate read-only
pass and surfaces *every* issue at once:

  $ secs_server --validate-config \
      --config equipment.yaml \
      --state-table control_state.yaml
  [error] equipment.yaml:5  svids[0].type — unknown SECS-II type `WTF`
  [error] equipment.yaml:7  alarms[0].category — value 200 out of range [0, 127]
  [error] equipment.yaml:9  host_commands[0].emit_ceid — CEID 999 not declared in `ceids` section
  3 error(s), 0 warning(s) across 4 files

What it catches:
- Missing required fields (device.model_name, .software_rev, …)
- Range violations (alarm category must be 0–127, spool streams 1–127,
  device.id fits u16, etc.)
- Unknown enum values (SECS-II types, HCACK values, control/PJ/CJ
  state and event names — using the right case + snake convention
  the runtime parsers enforce)
- Duplicate IDs within svids / dvids / ecids / ceids / alarms,
  duplicate PPIDs in recipes, duplicate command names in host_commands
- Referential integrity: host_commands[*].emit_ceid must exist in
  ceids; host_commands[*].set_alarm must exist in alarms;
  emit_on_control_change must exist in ceids
- PJ-table-specific: `NoState` sentinel rejected as `initial`,
  `from`, or `to` (matches loader's existing runtime check)
- yaml-cpp Mark → 1-based line numbers when available

What it doesn't catch (out of scope this round):
- JSON Schema for editor red-squigglies (future)
- Deep semantic checks across state-table reachability
- ECID min/max value parsing (would need numeric type coupling)

Tests cover: clean file passes; multi-error YAML surfaces every issue
on a single pass; line numbers populate; control_state /
process_job_state / control_job_state casing conventions;
format_issues_to renders both severities; the shipped
data/equipment.yaml etc. validate cleanly (regression tripwire if
anyone breaks the demo configs).

INTEGRATION.md §2.3 calls out the flag and suggests CI use.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CMakeLists.txt

@@ -64,6 +64,7 @@ add_library(secsgem
   src/gem/e84_state.cpp
   src/gem/host_handler.cpp
   src/config/loader.cpp
+  src/config/validate.cpp
   src/endpoint.cpp
 )
 add_dependencies(secsgem generate_messages)
@@ -142,6 +143,7 @@ add_executable(secsgem_tests
   tests/test_concurrency.cpp
   tests/test_thread_safety.cpp
   tests/test_persistence_upgrade.cpp
+  tests/test_config_validate.cpp
 )
 target_link_libraries(secsgem_tests PRIVATE secsgem doctest::doctest)
 target_compile_definitions(secsgem_tests PRIVATE