Merge driver for the sorted per-feature registries (dragon-sorted)

[thejackshelton]

Two PRs that each add one line at the same sorted place in a per-feature registry (for example #176's env and #179's ctx-proof in packages/parity/src/fixtures.ts) conflict in git's line merge, and the landing driver ejects the second PR, costing a whole round. This adds a git merge driver, dragon-sorted (scripts/sorted-merge.ts), modeled on dragon-floor.

What it does

• Runs git merge-file --diff3 -p of %O %A %B. A conflict hunk is resolved only when both sides keep every base line of the hunk, in order, and add only whole registry lines: a registry import (import { X } from './<dir>/<file>.ts';) outside the sorted block, or an entry of the sorted block ( id: X, / 'id': X,, or ...X_ANIMATION, in animation-kinds).
• Added lines are merged per gap by the key the registry's test sorts by: the id for entries (quoted and unquoted alike, so contexts < 'ctx-proof' < env); the file name with .ts for fixture-groups and regen-steps imports (as registry-claims.test.ts sorts them); the feature id for the codes, faults and animation-kinds imports (their existing order, which no test pins). A line both sides added appears once.
• Left as a conflict (exit 1, diff3 markers): a deletion, a modification, a key added with two contents, an added line that is not a registry line, a side whose own additions are unsorted, or a result whose import or entry list would not be strictly sorted. A file not in SORTED_REGISTRIES, a side without a final newline or with marker-like lines falls back to git's plain text merge, like floor-merge.
• Marked merge=dragon-sorted in .gitattributes: packages/dragon/src/css/animation-kinds.ts, packages/dragon/src/diagnostics/codes.ts, packages/dragon/src/faults.ts, packages/parity/src/fixtures.ts, scripts/regen.ts (exactly the central lists registry-claims.test.ts in both packages holds sorted, one line per feature).
• Registered by pnpm setup:git and in MERGE_DRIVERS (scripts/floor-merge.ts), which the landing driver's prepareWorktree already writes into the shared git config, so landing merges use it with no change to land.ts.

Tests

• New packages/parity/test/sorted-merge.test.ts: the env/ctx-proof case for imports and GROUPS lines (both orders give the same file); the same insertion on both sides; several interleaved additions from each side; a modification on one side and a deletion on one side left as conflicts (other hunks still resolved); one id added with two contents; a non-registry added line; refused inputs; the driver inside a real git merge (and the same merge conflicting without it); the CLI writes %A whole with no temp file left; .gitattributes marks exactly SORTED_REGISTRIES; each real registry parses (block found, every block line an entry, imports and entries sorted by its key, one import per entry, self-merge is identity).
• regen.test.ts merge-policy test: also skips the new merge=dragon-sorted lines (it already skipped dragon-floor; they are checked in sorted-merge.test.ts). No check removed.

End to end

Scratch worktree at origin/master (33efede) with the attributes committed and the driver configured, then git merge of origin/env-safe, origin/ctx-proof-rem, origin/ctx-proof-auto: all three merged with no conflict in any file; the last one printed sorted-merge: merged packages/parity/src/fixtures.ts (2 sorted hunk(s) interleaved) and gave contexts, 'ctx-proof', env in both the imports and GROUPS. With merge.dragon-sorted.driver=false the same last merge conflicts in fixtures.ts. Both registry-claims.test.ts files pass on the merged tree (18 tests).

Passed

• pnpm typecheck: pass
• vitest run packages/parity/test/sorted-merge.test.ts packages/parity/test/floor-merge.test.ts packages/parity/test/registry-claims.test.ts packages/dragon/test/registry-claims.test.ts: 52 passed
• vitest run packages/parity/test/regen.test.ts -t "merge policy": 4 passed
• Full pnpm test left to the landing driver (lane contract). No generated outputs change; no regen needed.

After this lands, run pnpm setup:git once per clone to register the driver (the landing driver does it itself).

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