ext/yac: Add a Memory Management chapter and improve the examples

[laruence]

This adds a Memory Management chapter and reworks the method examples.

Memory Management chapter

New chapter (memory.xml, four sections) explaining the two independent shared-memory pools:

• What Each Pool Holds — the key slot table vs. the value pool, embedded values, and a sizing guide for both pools
• The key pool (slots) — fixed-size slot table; expired and deleted slots are reused for free, only a probe path of four live entries triggers an eviction (least recently used, then least hit, then earliest probe), which is what the kicks counter actually counts; why slots_used == slots_size is not an error
• The value pool (segments) — 4M segments with a wrap-around bump allocator; a recycle overwrites the oldest values, whose reads fail the integrity guard and degrade to misses; the 1 MB stored-entry and 64 MB raw-value limits
• Sizing and what to watch — how to read the Yac::info() counters (interval hit rate rather than lifetime average; kicks, recycles, fails) and which knob to turn for each symptom

Examples

• Split the combined Yac::add / Yac::set / Yac::get / Yac::delete examples into one self-contained, runnable example per parameter form (single key, TTL, array form, default value, delayed deletion), verified against the extension
• Fix the Yac::dump() paging example, which printed int(100) on an empty cache (it returned an empty array), and correct the per-entry hits description (the counter resets on overwrite, not on delete or expiry)

Other changes

• Fill the empty constant descriptions (YAC_VERSION, YAC_MAX_VALUE_RAW_LEN, YAC_MAX_RAW_COMPRESSED_LEN) and fix typos in constants.xml
• Align the synopsis parameter names with the arginfo ($keys -> $key)
• Link the benchmarks section of the README from the introduction
• Document the compress_threshold clamping (1024..1M, -1 disables) and the -1 disable semantics in the ini description
• Drop the stale configure.xml template, which duplicated the installation section of setup.xml under the same xml:id

All examples were executed against the current extension build to verify their output.