ADFA-4128 (5/11): quickbuild:core — change detection & classification

[fryanpan]

Part 5/11 of the stacked split of #1669 (requested by Akash). Base: feature/ADFA-4128-qb-04-runtime. Stack overview + review mechanics: PR 1 (#1713). Terms are defined in quickbuild/README.md (lands in PR 1).

This is the first of several PRs for the Quick Build core that runs inside of Code on the Go.

Lets Quick Build notice every change a developer makes, however it arrives, so nothing ever builds stale.

flowchart LR
    save(["file write: editor save,<br/>git pull, Termux script"]) --> w
    subgraph s5["<b>This PR: core slice 1 — change detection</b>"]
        w["AndroidProjectWatcher (data)<br/>FileObserver + mtime poll<br/><i>AndroidProjectWatcher.kt</i>"] --> rec["WatcherBatchReconciler +<br/>trailing-debounce coalescing<br/>(domain/watch)<br/><i>WatcherBatchReconciler.kt</i>"]
        rec --> cls["ChangeClassifier (domain/classify)<br/>annotation-aware;<br/>assetsLiveReloadable gate<br/><i>ChangeClassifier.kt</i>"]
        ann["SourceAnnotationScanner<br/>(domain/annotations)<br/><i>SourceAnnotationScanner.kt</i>"] --- cls
    end
    cls -- "BuildRoute" --> orch["orchestration slice (PR 8)<br/>runs the route"]
    classDef thisPrBox fill:#dbeafe,stroke:#93c5fd,color:#1e3a5f
    classDef inPr fill:#ffffff,stroke:#64748b,color:#000
    class s5 thisPrBox
    class w,rec,cls,ann inPr

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What to review

• ChangeClassifier.kt — picks the cheapest still-correct route. The routing contract; line-by-line.
• AndroidProjectWatcher.kt, WatcherBatchReconciler.kt — watch rules, phantom-deletion guard, save-burst coalescing.
• SourceAnnotationScanner.kt — why an annotation edit is not just a code edit.
• Each package carries a README stating its contract — read those first.
• John's detection findings (C6, C7, C8, C20, C21) folded in as fixes.

How this PR Was Tested

• 17 test files, plus the RoomAppFixture and OfflineGuard network check.
• [verified 2026-08-21] At this cut: :quickbuild:core:test — only this slice's files exist yet, so the module suite is exactly the slice suite: 17 test files (16 suites; RoomAppFixture is a fixture), 221 tests per variant across all 6 variants, 0 failures, 0 errors. Coverage 97.4% line / 94.9% branch.
• End-to-end evidence: PR 11.

Coverage (JaCoCo at the stack tip, single run):

Package	Line	Branch	Note
…quickbuild.data	85.0%	60.0%	WatchService overflow paths need a real watcher
…quickbuild.domain	100.0%	100.0%
…quickbuild.domain.annotations	99.3%	97.3%
…quickbuild.domain.classify	99.2%	97.8%
…quickbuild.domain.watch	100.0%	94.3%
NON-UI TOTAL	97.4%	94.9%	734 lines, 506 branches

14 source files in the diff, all 14 measured.

Slice 1 of 4 — next: deploy and reload (PR 6).

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