docs: sync llms.txt with multi-generation engine changes

[LeonSGP43]

What

cd2f976 introduced Needle 2/3 engine generations (--generation flag on fetch/download, NEEDLE2_LIB_PATH/NEEDLE3_LIB_PATH overrides) and a strict mode for extract(), and updated doc/apis.md accordingly — but llms.txt was not fully synced. This PR closes the four gaps, keeping llms.txt consistent with the code and doc/apis.md:

#	llms.txt location	Gap	Fix
1	CLI summary — needle fetch	`--generation 2	3` flag missing
2	CLI summary — needle download <platform>	`--generation 2	3` flag missing
3	CLI summary — fetch air-gapped hint	Only mentions legacy NEEDLE_LIB_PATH	Now points at NEEDLE2_LIB_PATH/NEEDLE3_LIB_PATH (legacy var is a Needle 2 alias, deliberately ignored for Needle 3), matching doc/apis.md
4	Core API — needle.extract(...)	strict=True parameter missing	Added to signature plus one-line semantics from the docstring (temporal contradictions / engine-reported fabricated or negated values raise ExtractionValidationError)

Verification

Each edit is anchored in the code or doc/apis.md:

• needle/cli.py: --generation argument (choices=[2, 3], default=2) exists on both the fetch and platform-branch download paths.
• doc/apis.md: air-gap section documents NEEDLE2_LIB_PATH/NEEDLE3_LIB_PATH and the legacy-alias behavior; the API table already lists strict=True for extract.
• needle/__init__.py: extract(..., strict: bool = True) with the docstring quoted for the semantics line.

Scope

Documentation-only, llms.txt only — no behavioral changes, no other files touched.

Signed-off-by: Leon Gong leongong43@gmail.com