[02/08] replace commit identity storage with gix-backed aliases

[mjc]

Stacked on mjc/gitoxide-spike-01-gix-boundary.

Why this PR exists:

• PR 1 added the gitoxide and iddqd dependencies.
• This PR starts using them for commit identity storage.
• The old Oids structure stored raw git2::Oid values in a Vec and used a HashMap<git2::Oid, u32> for alias lookup.
• This PR moves the internal OID store to gix::ObjectId, stores alias records in an iddqd::IdHashMap, and keeps the public alias path centered on small u32 aliases.
• The app still has git2 call sites, so this is not a full gitoxide migration yet. It is the commit-identity storage layer that lets later PRs keep moving the graph/query code toward gix-backed data.

What changed:

• Replaces Oids internals with:
  • gix::ObjectId for stored object IDs.
  • IdHashMap<OidRecord> for OID → alias lookup.
  • Vec<ObjectId> for alias → OID lookup.
• Adds explicit git2 ↔ gix OID conversion helpers.
• Adds IntoGixOid so existing git2::Oid call sites can cross the boundary while the rest of the stack is migrated.
• Adds get_existing_alias for non-interning alias lookup.
• Adds get_alias_by_prefix so SHA-prefix navigation can find an existing alias without stringifying and reinserting OIDs.
• Adds reserve/shrink helpers for alias storage.
• Updates app, graph service, walker, worktree, stash, navigation, and reflog paths to use the new alias APIs instead of directly reading oids/aliases.
• Updates heatmap generation to build from sorted aliases instead of directly reading the old raw OID vector.

Algorithm difference:

• Commit identity is still represented externally by small aliases in graph/UI structures.
• The backing identity store is now gix::ObjectId plus typed iddqd records instead of git2::Oid plus a standard hash map.
• Existing git2 behavior is preserved by converting back to git2::Oid at remaining git2 call sites.
• SHA-prefix lookup now validates hex input and compares prefix nibbles directly against stored object bytes instead of allocating/stringifying OIDs.
• Heatmap counting now uses fixed-size count arrays for the 53-week grid and can also be fed through a streaming HeatmapCounts helper.
• Graph ordering, alias identity, stash/reflog/worktree lookup, parent/child navigation, and heatmap semantics are intended to remain equivalent.

Performance story:

• This should reduce OID churn in the identity layer by centralizing OID ownership and avoiding repeated direct access to raw git2::Oid collections.
• Prefix lookup avoids to_string().starts_with(...) scans and uses byte/nibble comparison instead.
• Heatmap counting now avoids an intermediate map and uses a fixed-size array for the rendered grid.
• No whole-app speedup is claimed here. Any runtime win should be measured in the later graph/query PRs.

Benchmark Results for this PR

Command shape: hyperfine --warmup 5 './target/release/guitar /home/mjc/src/linux --exit-when-graph-complete'.

CPU: M1 MacBook Pro 16GB

Ref	Total	vs main	User	System	CPU
02-gix-oids	1:14.26	2.12x	65.41s	2.17s	91%

CPU: 5950x Linux/ZFS

Ref	Total	vs main	User	System	CPU
02-gix-oids	1:10.85	1.40x	69.07s	1.78s	100%

Memory: 5950x Linux/ZFS

Ref	Peak heap	Peak RSS
02-gix-oids	1.38G	2.63G

Top 10 heaptrack allocation sites for `02-gix-oids`

1. 18210838 calls, 70.52M peak - git__calloc
2. 17431852 calls, 21.44M peak - git__substrdup
3. 6613871 calls, 18.44M peak - commit_parse
4. 4576967 calls, 0B peak - inflateInit2_
5. 4514697 calls, 265.92M peak - git__strndup
6. 4209519 calls, 279.42K peak - git_commit_list_insert
7. 2765451 calls, 366.68M peak - alloc::raw_vec::RawVecInner<>::finish_grow
8. 2096782 calls, 0B peak - git_commit_list_insert_by_date
9. 847866 calls, 472.20K peak - git_delta_apply
10. 484765 calls, 24.61M peak - alloc::sync::Arc<>::make_mut

[Copilot]

Pull request overview

This PR begins the migration of commit-identity storage from git2::Oid to a gitoxide-friendly representation by backing the alias store with gix::ObjectId and iddqd::IdHashMap, while keeping the external identity model as compact u32 aliases. It also updates SHA-prefix lookup and heatmap generation to avoid allocation-heavy OID stringification, and introduces test infrastructure to stabilize language/global state and git repo fixtures.

Changes:

• Replaced core::oids::Oids internals with gix::ObjectId storage + iddqd-backed alias records, with explicit git2↔gix conversion and a transitional IntoGixOid boundary.
• Reworked SHA-prefix navigation to validate hex input and match against OID bytes/nibbles (no to_string() scanning / reinterning).
• Updated heatmap counting to use fixed-size arrays (and a streaming HeatmapCounts helper), and adjusted graph/app/test call sites accordingly.

Reviewed changes

Copilot reviewed 33 out of 34 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/tests/helpers/localisation.rs	Uses stable tempfile paths and adds a language guard to prevent global-language test interference.
src/tests/helpers/keymap.rs	Adds language guard and removes manual language reset in keymap localisation test.
src/tests/helpers/heatmap.rs	Adds tests for new heatmap counting + streamed helper + prefix matching behavior.
src/tests/git/queries/remotes.rs	Switches test repos to shared init_repo_at fixture.
src/tests/git/queries/diffs.rs	Switches test repos to shared init_repo_at fixture and renames a local seeded init helper.
src/tests/git/actions/reverting.rs	Switches test repos to shared init_repo_at fixture.
src/tests/git/actions/rebasing.rs	Switches test repos to shared init_repo_at fixture.
src/tests/git/actions/merging.rs	Switches test repos to shared init_repo_at fixture.
src/tests/git/actions/cherrypicking.rs	Switches test repos to shared init_repo_at fixture.
src/tests/git/actions/branching.rs	Switches test repos to shared init_repo_at fixture.
src/tests/core/walker.rs	Updates tests to use new OID alias APIs (get_existing_alias, non-reference OID access).
src/tests/core/oids.rs	Adds focused tests for aliasing and prefix lookup without reinterning.
src/tests/core/graph_service.rs	Switches test repos to shared init_repo_at fixture.
src/tests/app/state/app.rs	Adds tests for wait_until_graph_complete success and error modes.
src/tests/app/input/navigation.rs	Switches test repos to shared init_repo_at fixture and adds language guard for language selection test.
src/tests/app/input/git.rs	Switches test repos to shared init_repo_at fixture.
src/tests/app/draw/settings.rs	Adds language guard to settings shortcut localisation rendering test.
src/main.rs	Adds --exit-when-graph-complete mode and CLI repo path handling for benchmarking/scripted runs.
src/lib.rs	Exposes new git::gix module and test-only git::test_support.
src/helpers/heatmap.rs	Replaces map-based heatmap counting with fixed-size arrays + HeatmapCounts, and adds sorted-alias heatmap builder.
src/helpers/branch_visibility.rs	Adds byte-slice refname parsing helper for gix-backed reference iteration.
src/git/test_support.rs	Introduces centralized test fixtures (repo init, commits, worktrees, submodules, temp paths, language guard).
src/git/gix.rs	Adds small gix integration helpers (object cache, commitgraph hint, branch tip iteration, error mapping).
src/core/walker.rs	Updates walker call sites to handle OIDs as values (no deref) after Oids API changes.
src/core/oids.rs	Implements new gix-backed alias store, conversion helpers, reserve/shrink helpers, and nibble-based prefix matching.
src/core/graph_service.rs	Switches heatmap building to sorted aliases and updates lookup paths to use new alias/prefix APIs.
src/app/input/worktrees.rs	Updates worktree selection logic to use get_existing_alias instead of direct map access.
src/app/input/navigation.rs	Updates parent/child navigation to use value OIDs and get_existing_alias.
src/app/input/modals.rs	Replaces SHA search via stringified OIDs with get_alias_by_prefix.
src/app/input/git.rs	Updates stash-related OID retrieval to new value-returning Oids API.
src/app/draw/stashes.rs	Updates stash rendering to use value OIDs (no deref).
src/app/app.rs	Adds bootstrap/shutdown helpers, adds wait_until_graph_complete, and makes sync resilient to graph channel disconnects.
Cargo.toml	Adds gitoxide/iddqd dependencies (gix, iddqd, etc.) and supporting crates.
Cargo.lock	Locks new dependency graph for gix/iddqd and transitive additions.

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