fix(merge): make --allow-plausible actually control track matching

[geoffmcc]

Summary

Fixes three defects in the three-way merge option contract. The headline one is that --allow-plausible did nothing.

1. --allow-plausible was inert

_analyze_tracks in src/alscan/merge/analysis.py hardcoded exact_only=False when building its internal matches, then separately computed an exact_only=True set and returned that. Two consequences:

• Plausible track matching was effectively always on for the analysis body, regardless of the flag.
• The returned match set (consumed by _analyze_track_order) was a stricter, different set than the one the rest of the analysis used.

Both the CLI and the GUI already exposed the option — src/alscan/gui/workers.py carries an allow_plausible field and the three-way page has a checkbox — so the setting appeared functional at both surfaces while having no effect.

Now a single match set is computed with exact_only=not inputs.allow_plausible and returned consistently. allow_plausible is threaded through ThreeWayInput and validate_three_way.

Behaviour change: default matching is now exact-only. Six tests in tests/test_merge_phase2.py now pass allow_plausible=True to retain their previous assertions.

2. Input role mislabel

validate_three_way iterated [(base_p, "base"), (ours_p, "theirs"), (theirs_p, "theirs")], so a malformed ours snapshot produced an error blaming theirs.

3. --output corrupted argument parsing

In _merge_guide, the --output branch was a bare pass that did not set skip_next. The output filename therefore fell through into clean_args and was parsed as a positional snapshot input. The value is now captured and the following argument skipped.

Also adds unknown-option rejection and a too-many-positional-args guard to both merge guide and merge plan, and removes a dead allow_unrelated_val assignment that was never read.

Tests

Adds tests/test_merge_option_contract.py (504 lines) covering exact-vs-plausible matching, input role labels, and guided-merge argument parsing.

Local verification mirroring ci.yml exactly, including -W error:

• pytest -W error --ignore=tests/gui — 791 passed
• QT_QPA_PLATFORM=offscreen pytest -W error -x --timeout=120 --timeout_method=thread tests/gui/ tests/test_services.py — 267 passed

Known gap

--output on merge guide is now parsed correctly but still never used — the value is captured and ignored, so no file is written. This is an improvement on the previous behaviour, which corrupted positional parsing, but the flag remains inert. Tracked separately in #25.