#2793 gatorwalk-factory: separate engine and tracker code behind a seam module the tests enforce

Part of the tracker-seam decision (https://claude.ai/artifact/TDidgyVjxKXU7JsNH1baNy): tracker functionality stays in gatorwalk-factory, but behind a seam the tests enforce, not a separate extension. Read gatorwalk-factory/DESIGN.md "Trackers" (:1092) first.

Today

No engine file imports tracker code (lifecycle.ts, work_item.ts, run_ops, run_store, gates, journal, work_item_ops, cel_context, metrics, summary and the report import none of tracker, tracker_methods, projection, claim, linear, swamp_club). Tracker code reaches into engine internals from many places:

• _lib/tracker_methods.ts:18-49: digestOf, parseRun, RUN_SPEC, payloadName, checkPinned, LIFECYCLE_SPEC, MethodOutput
• _lib/claim.ts:29-37: freshKey, loadHolderLifecycle, WORK_ITEM_TYPE
• _lib/projection.ts:17-24: journal types, ProjectionEntry, triggerKey, the template renderer
• swamp_club.ts:40-44, linear.ts:20: stringMapFrom, typeNameOf, MethodOutput

Nothing stops the next change from crossing the line in either direction.

Change

• Move files into _lib/engine/ and _lib/tracker/core/ plus _lib/tracker/backends/ (Lab and Linear clients and their fakes).
• Add _lib/seam.ts, re-exporting exactly what tracker code uses from the engine today. It is the only engine module tracker code may import.
• Add seam_test.ts: engine code imports nothing under tracker/, and tracker code imports from the engine only through seam.ts. The one current exception, fake_swamp.ts:18 importing claim types, moves to test support or goes through the seam.
• No behaviour change. Model types, method names and data names stay the same.
• DESIGN.md: a short "The seam" decision under "Trackers", saying what the rule is and why it is a test rather than a package boundary (no consumer besides gatorwalk; the tracker reads the run record, journal and pinned lifecycle, which a package split would turn into a cross-package API).

Order

After #2767 (template removal), which edits work_item_ops, lifecycle_schema and graph. Before #2771, #2772 and #2711, which edit tracker files. Every later issue in this series builds on it.

Done when

• The seam test passes, and fails on an added import that crosses the line in either direction (shown in the test).
• All existing unit and integration tests pass unchanged, apart from import paths.