[cache] Reject out-of-range keys on reads (#6419)

* [cache] Reject out-of-range keys on reads

An empty entry can match an out-of-range key when the packed cache
entry leaves storage bits unused. In the 256-entry binary coverage
cache this reports glyph IDs 32512-32767 as NOT_COVERED, skipping
Context/ChainContext format 2 substitutions and positioning.

Apply the same key range check in get() as set(). Add regression
coverage for spare-bit and full-width entries across all three
storage sizes, with and without atomics.

The new cache test fails before the fix and passes afterward, also
with HB_TINY. All 28 GSUB/GPOS Context/ChainContext boundary cases
pass. meson test -C build: 272 passed, 1 skipped, 0 failed.

Fixes: https://github.com/harfbuzz/harfbuzz/issues/6418
Assisted-by: OpenAI Codex

* [cache] Combine cache miss checks into one branch

Fold the key range check into the existing miss condition using
bitwise operations. XOR the stored and queried tags so their
mismatch can be combined directly with key overflow and the empty
entry check.

Use a scalar empty-entry constant rather than casting through
item_t, which creates temporary atomic objects in GCC builds.
The binary, mapping, and cmap lookup probes now each have one
conditional branch with GCC 15 and Clang 21 at -O2 on x86-64,
compared with two or three in the initial fix.

Testing: meson test -C build: 272 passed, 1 skipped, 0 failed.
All 28 GSUB/GPOS Context/ChainContext boundary cases pass. The cache
regression also passes with Clang and with HB_TINY.

Assisted-by: OpenAI Codex