feat: add diffBuffers, TreeBuilder, diffTreeToTree, findBlob and Diff.toPatch

[Brooooooklyn]

Closes #167

Issue #167 asks for diffing arbitrary strings like git diff --no-index. The reporter suggested TreeBuilder + diffTreeToTree as a workaround after finding git_diff_blobs is callback-based. This PR implements both the direct API and the workaround path.

New API surface

API	What it does
diffBuffers(old, oldPath, new, newPath, options?)	Repo-free diff of two raw buffers via git2::Patch::from_buffers. Returns { patch, hunks, stats } — unified patch text plus eagerly materialized hunks/lines (the Patch borrows the input buffers, so everything is copied out). null on either side = added/deleted file.
Diff.toPatch()	Render any Diff as unified-diff text, including sigils (+/-/space) matching libgit2's diff_print.c.
repo.treebuilder(source?) → TreeBuilder	insert(name, oid, filemode) / remove / get / write / clear / len / isEmpty — the reporter's literal request.
repo.diffTreeToTree(oldTree?, newTree?, options?)	Tree-to-tree diff; null side = empty tree.
repo.findBlob(oid)	Look up a blob by OID (new BlobParent::Repository variant).

Reporter's use case, now possible

import { diffBuffers } from '@napi-rs/simple-git'

const { patch } = diffBuffers(
  Buffer.from(expected), 'expected.txt',
  Buffer.from(actual), 'actual.txt',
)

The synthetic-tree route also works end to end: repo.blob() → treebuilder.insert(..., 0o100644) → write() → diffTreeToTree → toPatch().

Tests

• __test__/buffer-diff.spec.mjs — identical/modified/null-side buffers, stats, toPatch on a workdir diff
• __test__/tree-builder.spec.mjs — builder round-trip, findBlob, the issue-167 two-tree diff flow
• Full suite: 188 tests pass, no regressions

---

Note

Medium Risk
Additive native surface with one unsafe libgit2 patch call; behavior is well-tested but touches ODB writes and diff output consumers may rely on exact patch bytes.

Overview
Adds in-memory and tree-based diffing (closes #167): compare arbitrary byte buffers like git diff --no-index, build synthetic trees in the ODB, and emit standard unified patches.

diffBuffers is a repo-free entry point that returns unified patch bytes, structured hunks/lines, and stats. It calls git_patch_from_buffers directly so null buffers mean absent files (/dev/null, add/delete) rather than empty content; results are fully copied out for JS.

Diff.toPatch() prints any existing Diff as raw patch bytes, prepending +/-/space sigils to match libgit2’s patch format.

On Repository: treebuilder(source?) + TreeBuilder (insert/write/etc.), diffTreeToTree, and findBlob(oid) (new BlobParent::Repository). Together they support blob → single-entry tree → tree diff without touching the workdir.

Also wires libgit2 init at native module load and adds AVA coverage for buffer diffs, tree builder round-trips, and workdir toPatch.

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[samvv]

Thank you!