* content: add delta-chain depth benchmark for #85 re-measurement

BenchmarkExpandOldestVersion expands the oldest version of a ~26 KB, 400-line file at 12/50/200 revisions, mirroring the table in #85. The chain is built through content.Deltify so the on-disk shape matches what content_deltify produces: newest version whole, each older version a delta against the one newer.

Reports ns/op plus B/op and allocs/op so the per-link allocation profile the issue is about is directly observable.

* content: straddle delta.Apply's 64 KiB cap in the chain-depth benchmark

BenchmarkExpandOldestVersion measured a ~26 KB file, which sits below delta.Apply's deltaInitialCap: for targets that small the output buffer is allocated at exactly targetLen and the capped/growOutputCap path is never entered, so the arm cannot observe #81/#82 at all.

Parameterise the chain builder on file size and add a ~206 KB arm above the cap, so the two arms isolate the threshold rather than leaving it inferred from reading delta.go. Also move the chain-depth metric after the loop -- ResetTimer deletes user-reported metrics, so the depth reported before it never reached the output.

Refs #85

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Co-authored-by: Dan Mestas <[email protected]> check-in: c3338c4d38 user: [email protected] tags: trunk