feat: TRAC-925 upgrade: whole-tree merge engine and index staging

[chanceaclark]

Jira: TRAC-925

PR 2 of 4 for catalyst upgrade (LTRAC-956). Stacks on TRAC-924 (#3063).

What/Why?

Adds the whole-tree merge engine (git merge-tree --write-tree) and the index staging layer (applyIndexState).

The whole-tree engine is the default (used when git >= 2.38). Key design decisions:

• Three sides (base/ours/theirs) are snapshotted as commits in a throwaway object store with separate GIT_INDEX_FILE per side — this lets all three git add -A calls run in parallel without index races, while still sharing the object store for blob dedup
• git merge-tree -z --name-only output is NUL-delimited: <tree-oid>\0<conflicted-path>\0...\0\0<messages>. The empty field after the last conflicted path is the terminator; everything after is informational and ignored
• checkout-index only writes, never removes — deleted paths are identified via git diff --no-renames --name-status oursTree mergedTree and removed separately
• NULL_OID is detected via git rev-parse --show-object-format to support SHA-256 repos (OID = 64 hex vs 40). The OID regex already accepts 40–64; NULL_OID now matches

applyIndexState registers conflicted files as real unmerged index entries (stages 1/2/3) so VS Code's merge UI activates. MERGE_HEAD is intentionally absent — resolving is a normal git add + commit, not a merge commit.

Testing

pnpm test upgrade.spec

Covers: resolveStrategy, applyIndexState, mergeCore engine: tree × 7 (same contract as per-file — both must agree), mergeCoreTree rename fidelity.

Migration

No user-visible changes yet.

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Project	Deployment	Actions	Updated (UTC)
catalyst	Ready	Preview, Comment	Jun 26, 2026 7:01pm

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Bundle Size Report

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Unlighthouse Performance Comparison — Vercel

Comparing PR preview deployment Unlighthouse scores vs production Unlighthouse scores.

Summary Score

Aggregate score across all categories as reported by Unlighthouse.

	Prod Desktop	Prod Mobile	Preview Desktop	Preview Mobile
Score	89	93	92	95

Category Scores

Category	Prod Desktop	Prod Mobile	Preview Desktop	Preview Mobile
Performance	75	88	76	81
Accessibility	95	95	95	92
Best Practices	100	100	100	100
SEO	88	88	100	100

Core Web Vitals

Metric	Prod Desktop	Prod Mobile	Preview Desktop	Preview Mobile
LCP	3.3 s	3.8 s	3.6 s	5.1 s
CLS	0	0	0.001	0
FCP	1.4 s	1.2 s	1.2 s	1.1 s
TBT	0 ms	0 ms	0 ms	0 ms
Max Potential FID	30 ms	40 ms	40 ms	50 ms
Time to Interactive	3.3 s	3.9 s	3.6 s	5.1 s

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