support multiple firmware revisions per terminal in YAML

[gilesknap]

Summary

Fixes #60. Some Beckhoff terminals rename PDOs across revisions while keeping the same (vendor, product). EP2338-0002 renames RxPDOs Channel 1..8 → Channel 9..16 at revision 0x00120002, so rigs at that firmware that loose-matched the 0x00100002 YAML fabricated wrong ADS symbol names and AO writes silently failed.

Lookup change in get_terminal_type_by_identity:

1. Exact (vendor, product, revision) match.
2. Highest (vendor, product) candidate with revision ≤ rig.
3. Lowest-revision (vendor, product) candidate (degraded fallback) with a warning.
4. None.

Schema: Multi-revision entries use the suffix __rev<8 hex> (lowercase rev, no @ — that character would flow into GUI DOM IDs). The bare-id entry stays the lowest cached revision, so existing deployments don't need a key rename. service_file.merge_xml_for_terminal strips the suffix before fetching the ESI XML and threads the entry's pinned revision through as target_revision so the parser picks the right <Device>.

Adds EP2338-0002__rev00120002, picking up the renamed Channel 9..16 RxPDOs from the cached XML. The bare EP2338-0002 entry is unchanged.

GUI rework and the merge-idempotency follow-up are deferred to later slices.

Test plan

• Unit coverage in tests/test_symbol_expansion.py exercises exact/highest-≤/degraded-fallback paths.
• Rig verification on EP2338-0002 at firmware 0x00120002 (AO writes land at the right Channel 9..16 symbols) — pending.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.28%. Comparing base (9fddb4a) to head (80d43a6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   75.26%   75.28%   +0.01%     
==========================================
  Files          19       19              
  Lines        4096     4098       +2     
==========================================
+ Hits         3083     3085       +2     
  Misses       1013     1013              

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