[RNE Rewrite] Extract shared host-object try-lock helper into core

[msluszniak]

Description

Resolves the locking + boilerplate items of #1316. Extracts the try-lock + liveness-check block that was duplicated across TokenizerHostObject (5 sites), ModelHostObject (3 sites) and the tensor helpers into a single templated core::tryLock helper (cpp/core/host_object_lock.h), parameterized over the lock kind (std::unique_lock / std::shared_lock) and a liveness predicate. Error messages are preserved verbatim.

On the mutex question from #1316: TokenizerHostObject intentionally keeps an exclusive std::mutex rather than moving to std::shared_mutex. Although every tokenizer op is const, const is not thread-safe here — HF tokenizers encode via the PCRE2 fallback, and the vendored Pcre2Regex shares a single pcre2_match_data member across const find_all() calls, so concurrent encodes on one tokenizer would race. This is documented in tokenizer.h with a TODO pointing at follow-up #1330 (switch to shared_mutex once the vendored tokenizers lib includes the upstream per-call fix).

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

Pure refactor with no behavior change: the extracted helper reproduces the exact same lock discipline and error strings as the inlined code. Verified the changed translation units compile (clang++ -std=c++20 -fsyntax-only) and pass clang-format.

Screenshots

Related issues

Resolves the locking/deduplication parts of #1316. Follow-up: #1330.

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes

host_object_lock.h is a header-only template, so no CMake change is needed (sources are globbed and it is included where used).