fix(workingdiff): diff committed trees with one git process

[akonwi]

Fixes #68.

Problem

Observing a commit or branch diff target read both endpoint trees in full, starting one git cat-file tree process per directory. Cost scaled with repository size rather than change size: in this repository (306 directories) two walks took ~8.4 s against the 8 s observation budget, so every commit target failed with context deadline exceeded. Reading a file from a committed observation re-walked both full trees as well.

Change

• diffCommittedTrees runs a single git diff-tree -r -z --raw --no-renames --ignore-submodules=none <base> <head> and returns only changed entries. The empty-tree base uses git's well-known empty tree OID for the repository's object format.
• The call is bracketed by validateObjectStoreAuthority, the same before/after loose-object and pack check mergeBase uses, so unsafe object storage is still rejected.
• revalidateCommittedFile (file reads) uses the same comparison restricted to the requested path.
• Downstream classification is unchanged; it already only consumed differing paths.

Behavior notes

• Tree object bytes are no longer re-hashed against their OIDs; git's diff-tree output is trusted, as ls-tree already is for working-tree observation.
• Unsupported paths are counted only among changed paths, so invalid paths in untouched directories no longer mark an observation incomplete.
• Repository size no longer triggers the traversal limit; only a change whose raw diff exceeds 16 MiB fails, currently as a generic repository_unavailable rather than limit_exceeded.

Validation

• New regression test: a repository with 300 nested directories and a one-file change; observe and read each run exactly one diff-tree and at most 10 git processes, counted by a git wrapper on PATH. The test fails on main with context deadline exceeded.
• Parser tests for added, deleted, modified, symlink, file-to-submodule, and unsupported-path records, plus rejection of malformed records.
• Against this repository, every commit and branch target observes in ~110–270 ms.
• gofmt -l ., go build ./..., go vet ./..., go test ./..., go test -race ./internal/workingdiff/.

#69 and #70 (error propagation for diff failures) are not addressed here.