tests: variable-length parametrize for 2KB-budget regression test

[tony]

Summary

The existing 2KB-budget regression test parametrized (3 tiers × 2 tmux_pane states), but every case used %42 (3 chars) and default (7-char socket name). A user with a slightly longer custom socket name + a multi-digit pane id from a long session pushes the readonly worst case very close to 2048 bytes, and the static cases never exercised this — future text additions could silently put realistic runtime injections over the budget.

This PR:

• Collapses the two cross-product @pytest.mark.parametrize decorators into one explicit list so the variable-length stress case is a peer entry instead of expanding the cross-product space
• Adds (TAG_READONLY, "%99", "/tmp/tmux-1000/dev-prod,12345,0") as the 7th case. Exercises BOTH axes: multi-digit pane id and a longer-than-default socket name. Margin ~2 bytes from the 2048 ceiling.
• Inline comment names the fallback path (tighter compression form) if a future text addition trips this case.

Note on base branch

This PR targets pane-discoverability (PR #37), not main. The variable-length stress test cannot pass off main alone — the 2KB compression in pane-discoverability is what makes the budget fit in the first place. Merge after #37 lands.

Test plan

• uv run ruff check . && uv run mypy && uv run py.test -q (443 passed; +1 stress case)
• just build-docs

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.91%. Comparing base (a3ff362) to head (52c849e).

Additional details and impacted files

@@                  Coverage Diff                  @@
##           pane-discoverability      #42   +/-   ##
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  Coverage                 84.91%   84.91%           
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  Files                        40       40           
  Lines                      2294     2294           
  Branches                    294      294           
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  Hits                       1948     1948           
  Misses                      261      261           
  Partials                     85       85           

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[tony]

Folded into #37 per the multi-model weave-ask synthesis (2-of-3 votes for fold; the variable-length parametrize is intrinsic to #37's 2KB-budget contract, not an independent feature).

The variable-length stress case (%99 + dev-prod socket, margin ~2 bytes from 2048) now lives in the same commit that introduces _INSTR_SCOPE and the budget regression test on pane-discoverability — see 4d5744e mcp(refactor[server]): Compress instructions, add SCOPE, fit under 2KB transmitted.

Closing without merge.