PR-size check in PR CI: changed lines per PR, derived files excluded (p90 ≤200 gate) #5733

Refs #5498, #5510. Ordered by aprender-57 (operator plan step 5): every aprender PR is ≤200 changed lines, one ticket per PR; the gate is p90 PR size ≤200.

Loss

Nothing in PR CI measures PR size. Fold PRs and large PRs get past review sizing, hold seats longer and conflict more (#5732 is 12 files / 507 changed lines; the README-count fold conflicts in #5723/#5724/#5727/#5728). #5498 states the ≤200 target but has no check behind it.

Change (workflow change; owner aprender-flow implements)

Add a PR-size check to PR CI: count changed lines (additions + deletions) of the PR diff, excluding derived files (contracts/witness/*.json, contracts/contracts.nt, contracts/shapes.ttl, ci/ fragments, README counts), and report the number. Failing threshold and the p90 rollup are the flow owner's call at implementation.

Falsifiers (test first)

• A fixture diff of 201 changed non-derived lines: the check FAILs and prints the count.
• A fixture diff of 200 lines: PASS.
• A diff of 5 source lines plus 500 derived-file lines: PASS (derived excluded).
• Control: the check run against a PR labeled as a squash fold of two tickets reports the sum, not the largest part.

Process

Gate/workflow change: C324 rule 7 applies (GATE only with the operator's yes, one gate retired per gate added). Quorum + non-author "gate/workflow sign-off: YES" on the final commit, 2 seats. Milestone 0.72.0 (EPIC 20 step 1).

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