fix(prewarm): skip suffix fetch for fully resident sources

[orut34iop]

A fully prewarmed file still starts a speculative suffix request on open: adoption installs the whole head, but the warm.tail == nil check treats the missing separate tail span as missing bytes. A 40 KiB source is fetched again by Range: bytes=-65536 even though its entire content is resident. The existing immediate request-count assertion can pass before that asynchronous request reaches the origin; this caused the all-traits failure observed on #823.

Skip tail prefetch only when the adopted head covers its declared content length. Partial warms, cold opens, header identity checks, redirect handling and memory-pressure eviction retain their existing paths. No public API or dependency change.

Strengthen the full-warm test with a bounded observation of absence, covering files smaller and larger than the suffix budget. Add a positive witness that partial warming still requests the missing suffix. The existing redirect/header-mismatch coverage runs alongside them.

Upstream context: #551, #554 and #584 already provide prewarm and resolved-target adoption; this fills the complete-head boundary they leave open. The affected production and test files are identical on main 151322d1b843b4414a0fcffa9ed44e8b3a9d087e and #823. This independent fix does not alter #823's audio coverage scope.

Validation on Apple Silicon Mac17,3, macOS 26.7.1 / Swift 6.4:

• Before the fix, the strengthened witness fails with two requests instead of one, and the second raw Range header is bytes=-65536.
• 56 prewarm/redirect tests in 9 suites pass under the strict cooperative pool, including both complete-head sizes and the partial-warm positive case.
• Full serial default-traits run: 795 XCTest tests passed (1 existing skip); 4,615 Swift Testing tests ran with three assertions failing in the existing SoftwarePacketMemoryFIFOTests.evictPurgedHistory test (lines 99/102/103). Its consumed-history residency assumption is exactly the issue addressed by open test(cache): respect kernel purges in history eviction checks #819. This PR changes neither that test nor the memory FIFO; the isolated 11-test memory FIFO suite subsequently passed without changes. The full-run failure is retained and is not reported as a full-suite pass.
• Documentation links and git diff --check pass. Host executables are signed and verified with a valid Apple Development identity; linker ad-hoc signing is disabled.

Other platforms and all-traits coverage are left to CI. No new device playback acceptance is claimed.