Restore parent-session Files Changed invalidation

Acceptance Criteria

• A task tool part records its { parentSessionId, sessionId } relation before the child’s file-edit event arrives.
• A completed child edit, write, patch, or apply_patch event carrying a file diff and delivered through the live server listener increments the child and every recorded task-spawn ancestor diff-version in ServerSession.
• For root --session_spawn--> tab --task_spawn--> worker, a completed worker edit increments worker and tab exactly once, leaves root unchanged, and changes the viewed parent query key from busy:0 to busy:1 before any parent idle event.
• A child with only generic session parentage increments only its own diff version.
• Directory data.diff_version and set("diff_version", ...) both address the ServerSession record; the directory cache has no independent diff-version map.
• Directory trimming and deletion do not access a diff-version field absent from the directory cache shape; ServerSession eviction removes the evicted session’s shared version entry without error.
• The recorded chore: fix pre-existing main breakages blocking clean overlay composition (#821) #832 known-fix contract replaces the obsolete directory-cache evidence with identical shared-state evidence in both the machine registry and its human-facing fixture list: initialized ServerSession diff versions, task-spawn ancestor invalidation, and shared eviction cleanup. It rejects omission of any required shared protection or restoration of a directory-owned diff-version copy.

Testing Decisions

• Extend the real serverSDK.event.listen route with a task tool relation, a child file-edit event, and a busy parent.
• Extend the Files Changed query test so the parent refresh occurs before its idle status event.
• Exercise directory trimming/deletion against a cache shape with no diff-version map and ServerSession eviction against the complete shared state.
• Preserve reducer-unit coverage, but do not treat it as sufficient proof of the production route.
• Exercise the shared known-fix registry and its human-facing contract in both positive and negative forms, then run the packaging build path that consumes the registry.
• Extend the Enforce known fixes during CI packaging #1469 diverged-CI=1 packaging fixture: all replacement protections stage with a recorded advisory; omission of each replacement protection fails before staging, names KNOWN-FIX MISSING and #832, emits no advisory, and leaves both staged artifacts absent.

Key Decisions

• ServerSession owns diff-version initialization, mutation, task-spawn ancestry, and shared eviction.
• Only task tool metadata authorizes parent invalidation; generic parentID, filesystem observation, and status transitions do not.
• This slice must land before PR fix(app): the switch-latency arc — mount, mirror, warm, probes + snapshot-first boot (#1306) #1360 is rebased or reconsidered.
• A known-fix contract protects the current canonical behavior and ownership boundary; it must not fossilize superseded internal storage syntax.

Constraints & Invariants

• Do not remove diff-version state or cast around a missing cache field.
• Do not use parent status transitions, polling, or filesystem watchers as the primary live-refresh mechanism.
• Do not modify Fleet v2 provenance behavior.
• Do not weaken known-fix enforcement, suppress the packaging guard, or widen Enforce known fixes during CI packaging #1469; migrate only the chore: fix pre-existing main breakages blocking clean overlay composition (#821) #832 evidence from obsolete directory-cache syntax to the verified shared-state behavior.

Prior Art

• The task-spawn Files Changed rollout and parent invalidation repair.
• The current session cache contract and production event-routing tests.

Source

• Part of Recover session reliability regressions without merging PR #1360 #1465.
• Blocks rebase review of PR fix(app): the switch-latency arc — mount, mirror, warm, probes + snapshot-first boot (#1306) #1360.