refactor(session): one watch plan, registered by every host

[codecaaron]

Gut check C ("one watch plan") and B4. Opened as a single PR for Linux CI, since it changes what each host registers with its watcher. spike-codex is reviewing the same range locally.

What changed

• Red first 053d228 (B4): the Turbopack and CLI orchestrator watches an absent system path outside the root, and creating it reloads the system. Red on main 0e9ceda: no watcher is opened for it.

• One watch plan 485ff93: ExtractionSession.watchPlan(covered) returns one plan.

  • files: the system module and the modules it read, asset sources, configuration inputs.
  • missing: the paths the system load found absent, less any inside the host's covered recursive roots.
  • directories: the canonical external roots.

  Hosts keep only registration:

  • Vite: chokidar add, each path once. The root and the external directories count as covered, so no absent file is watched inside a watched directory (the inotify finding).
  • Next webpack: webpack's file, missing and context dependencies.
  • unplugin: Rollup's addWatchFile over its analysed files plus the plan's files and absent paths, with no directories, since Rollup's graph covers imported kit sources.
  • Turbopack/CLI orchestrator: dependency watchers.

Host-visible differences

• Config inputs (package.json, the tsconfig chain) are now watched by Vite, Next webpack and unplugin (unplugin used to watch only the root tsconfig.json).
• Vite also registers asset sources.
• The orchestrator watches absent system paths outside the root.

Stated limit

The coalescing scheduler stays per host. The mechanism is already one CoalescingWindow. Moving the windows into the Session adds about as much as it deletes and shifts Linux timing, and Vite's system reload needs per-window outcome scoping.

Proof

• Red-first row above.
• 707 targeted tests (Session, Vite, unplugin, Next, CLI).
• All 13 local macOS tiers green on 485ff93.

Review fix (spike-codex's P1, silent stale CSS)

• The P1: a plugin instance reused with a replacement dev server kept its registration memo, so outside-root paths were never registered with the new server's watcher.
• Fix: b178dcb keys the memo by watcher.
• Witnesses, both red on 485ff93 and green on b4cabf8:
  • 16365c6: a registration row, where a replacement watcher gets every path again;
  • b4cabf8: a real Vite witness. One plugin object, server 1 closed, server 2 started, and an outside-root palette edit must reach server 2's CSS.

Further stated limits (from the review)

• Absent config inputs: they sit in the plan's files, so the covered-root filter (which applies only to missing) doesn't screen them. Vite therefore adds an absent in-root tsconfig.json to its watcher. No CSS failure was shown.
• Config edits under Vite: registration doesn't route them. Vite's hot-update path ignores .json and checks only system dependencies, so a package.json edit doesn't reach the Session. This routing predates this PR.

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