fix(s3lite): bound ranges and serve HEAD from retained metadata

[sorenbs]

Ranged HEAD requests now use retained object metadata without indexing a body discarded by the emulator's benchmark mode. Range parsing returns bounded, nonempty usize slices with checked wire-integer conversions. Zero-length suffixes use the existing invalid-range full-response fallback; other valid and invalid range behavior stays covered by explicit vectors.

Addresses five inherited warnings, including a narrow poison/header invariant contract on the GET snapshot owner. Both active platform inventories and the source ceiling are pruned; immutable adoption inventories are unchanged. Targets slate and includes #63 with its prerequisites.

The old source fails the discarded-body HEAD regression with a slice panic and returns the wrong status for the zero suffix; a valid-range control passes. The corrected release build passes all four binary tests, including 1,024 generated range cases and exact u64::MAX boundary vectors. Pinned workspace Clippy and both ratchets pass (3,517 warnings on this minimal branch), as do the common quality gate (12 syntax/47 Python controls), 52 invariant tests at 1,024 property cases, and 11 service invariant tests. Full-target verification planning selects compiler, properties, Loom and Miri; it finds no critical mutation scope. All 13 exact-head CI checks passed or were intentionally skipped (the noisy campaign); ready for review.

This preserves this emulator's invalid-range fallback; it does not claim full S3 Range compliance.