[leehack]

Adds int32_t litert_lm_native_in_process_litert_abi_version(void) to the LiteRtLmBridge exports on every platform, so consumers can tell runtimes whose LiteRT-LM → LiteRT external-weight handoff is ABI-valid from the v0.16–v0.19.1 Linux/Windows hosts that load upstream's prebuilt libLiteRt (#111). Today a consumer can't: v0.19.0, v0.19.1 and v0.19.1-1 all report upstream C API 1.0 (llamadart#1795).

Contract

Value	Meaning
1	LiteRT compiled into the runtime; the build verified the binary imports no separate libLiteRt
0	Not established (every build before v0.19)
missing	Runtime predates the marker; treat as 0

• Every v0.19+ source build reports 1 on all platforms. Unknown future values must be rejected.
• Additive bridge export only; no existing symbol, signature or upstream C API changes. It is not an upstream C API minor bump: litert_lm_get_api_version stays upstream's.
• Releases through v0.19.1-1 lack it. v0.19.1-1 already links LiteRT in-process, so a consumer that must accept it identifies it another way (e.g. a pinned checksum).

Documented in docs/platform_strategy.md.

Changes

• Bridge: new export, selected by --define=litert_lm_in_process_litert=true (→ LITERT_LM_IN_PROCESS_LITERT), which build_upstream_runtime.py passes for v0.19+.
• The linkage check now covers every platform the marker claims: ELF (Linux, Android), PE (Windows) and Mach-O (macOS, iOS, including LiteRt.framework). Before, it covered Linux and Windows only. Published v0.19.1-1 Android, macOS and iOS hosts were inspected and import no libLiteRt.
• litert_lm_finish_reason_smoke.py --abi-only also checks the marker value of the built Linux x64, macOS arm64 and Windows x64 runtimes in PR CI.
• Follow-ups from #112 audit (desktop in-process LiteRT) #113 item 4: main()'s duplicated validation calls move into validate_runtime(). A new test fails on both build paths if the linkage check is dropped.
• Follow-ups from #112 audit (desktop in-process LiteRT) #113 item 6: the CPU options decision message no longer points future tags at the prebuilt libLiteRt.

Validation

• python3 -m unittest discover -s tools -p 'test_*.py': 445 OK, 3 skipped. diagnostics tests pass.
• Mutation checks:

Mutation	Result
Drop the linkage call from validate_runtime	2 failures
Drop either validate_runtime call in main()	1 failure each
Rename the Bazel define	16 failures
Return 0 from the marker under LITERT_LM_IN_PROCESS_LITERT	C bridge test fails

• Full native matrix: PR qualification on this head.

Refs #113. Items 1, 2 and 5 stay open (see the comment on #113). Item 3 was done in #114.