refactor(product): mark_active_param takes only the ref_id

[kewde]

Summary

Follow-up to #62. EvalContext.mark_active_param took both a
ParameterRef id and the Parameter/UnionParameter id it targets,
with the caller (ParameterRefRefNode) responsible for resolving the
latter itself. Since EvalContext already holds the
ApplicationIndexer, and ApplicationIndexer.parameter_refs already
maps ParameterRef.id -> ParameterRef.ref_id (the id it targets), the
resolution belongs in EvalContext itself rather than at every call
site.

• mark_active_param(ref_id) now takes a single argument and resolves
  parameter_id = self._idx.parameter_refs[ref_id].ref_id internally.
• EvalContext.idx is now a required, keyword-only argument instead
  of ApplicationIndexer | None with a dead-branch RuntimeError in
  allocate() - every real construction path already supplied a real
  indexer, so the None case only existed in tests.
• All ~60 test call sites across packages/product/tests/parser_v2/
  now pass idx, via a new shared idx pytest fixture
  (tests/parser_v2/conftest.py) for tests that don't care what's in
  the indexer, or a locally-built one for tests (test_context.py,
  test_module.py) that already had their own.
• Removed test_allocate_without_indexer_raises, which tested the
  idx=None path that no longer exists.

No behavior change: encode_to_memory() output for the
gira_2gang_button_interface.knxprod fixture is still byte-identical
before and after this change.

Test plan

• uv run pytest packages/product (536 passed)
• uv run ruff check / uv run ruff format --check
• uv run pyright packages/product (same 30 pre-existing errors as base branch, zero new)
• encode_to_memory() output byte-identical against the real fixture, before and after
• apps/knx-gui imports and its test suite pass unaffected

🤖 Generated with Claude Code

https://claude.ai/code/session_019yPeudwyfcjBGgTNN3Ad7p

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.47%. Comparing base (22a6b96) to head (dfad269).

Additional details and impacted files

@@                           Coverage Diff                           @@
##           fix/dynamic-activity-gates-encoding      #63      +/-   ##
=======================================================================
- Coverage                                59.48%   59.47%   -0.01%     
=======================================================================
  Files                                      137      137              
  Lines                                    10398    10397       -1     
  Branches                                  1432     1431       -1     
=======================================================================
- Hits                                      6185     6184       -1     
  Misses                                    4081     4081              
  Partials                                   132      132              

Files with missing lines	Coverage Δ
.../product/src/xknxmono/product/parser_v2/context.py	100.00% <100.00%> (ø)
...xmono/product/parser_v2/nodes/parameter_ref_ref.py	100.00% <100.00%> (ø)
...es/product/src/xknxmono/product/parser_v2/state.py	100.00% <ø> (ø)

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