Fix created/last updated dates picked up by the walker

[rumblefrog]

Symbol created / last_updated metadata was wrong in several ways. Some bugs were in libwalker, others in how chumbucket merged symbols.

libwalker

• Root commit skipped. Only commits with exactly one parent were diffed, so anything in the initial commit was first "created" at its next modification. The root commit is now diffed against an empty tree.
• Merges skipped, side branches interleaved. The walk visited every commit sorted by committer time and skipped merges. Side-branch commits were applied between mainline commits that didn't contain them, so symbols were added, then "deleted", and dated wrong, and anything that only landed via a merge was never seen at that point. The walk now follows first-parent history and diffs merges against their first parent, so a symbol is dated when it landed on the branch.
• Incremental resume by time. It now resumes from the bundle's last commit hash, falling back to committer time if that commit isn't in the history.
• count was the 0-based index in a time-sorted walk. It is now the real git rev-list --count for that commit, which is the SourceMod build number.
• An unreadable entry (for example a submodule) or a commit with no readable blobs used to end the walk silently. Those are now skipped.

chumbucket / schema

• The <<= operator on methodmaps, enum structs, enums and typesets replaced their nested members wholesale. So whenever a container changed, every method, property, field, entry or type was reset to created = last_updated = current commit. Nested members are now merged individually, keeping their metadata.
• Entries without metadata now get it even when they differ. Additions and removals count as diffs again, so a commit that only adds or removes symbols writes the bundle.

Verification

• New libwalker/tests/history.rs builds a repo with a root commit, a merge with an out-of-order side branch, and a checkpoint. It checks the commits, paths, contents and rev-list counts. Added it to CI.
• End-to-end run of chumbucket on a synthetic repo, full and incremental (with -b). Metadata was correct for top-level and methodmap members. The old binary misdated the root-commit symbols and took "last updated" from an unmerged side commit.
• Full SourceMod run (7457 commits, about 5s in release): GetClientAuthId is created at a7cdf17 (the PR #147 merge, count 4830), and the head count is 7457. Both match git log --first-parent -S and git rev-list --count.

Existing bundles hold the old dates and counts, so they should be regenerated from scratch (without -b).

🤖 Generated with Claude Code

https://claude.ai/code/session_01UmDZ8MBYNvgeQ52tpsJe3t

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