native: a CUDA family's prompt cache, weight share and open errors come from its own declarations

[Bizuayeu]

What this changes

The CUDA host wired Nemotron's prompt-cache copies, rules and budget in by name. They now come from the family's
snap_* declarations through cuda_snaps.zig: a family without snap_save gets no prompt cache, snap_ready sets
aside its state buffers, and snap_grid makes the store keep states only on the family's chunk starts (for a family
whose rows depend on where a prompt's chunks begin). A family that declares warm_quiet gets 1.0.9's warm turns
(.warm = .waits). Nemotron declares warm_quiet and no grid and keeps 1.0.9's rules: any row, the head one
token on, warm turns, the same budget.

Three more hooks for a family split over hosts: weightShare (the weights this rank holds, for the memory check),
explains_open (the family's own text for an open error) and the learner only for a family with slide. The
GLM-5.3-Flash family (#548) uses all of them.

Receipt

• Environment: TensorFold 1.0.9 (c36b536d) plus this change; one GB10 (DGX Spark, sm_121, CUDA 13.3).
• Exactness: Nemotron's open, prompt cache and --slide are unchanged (the same calls and rules, now through the
  family's declarations).
• Decode and prompt speed: not applicable.
• Tests run:
  • zig build test-cuda-host: 430 passed, 0 failed; the new "a family with snap_grid keeps on its chunk starts, any
    other at any row as Nemotron does" needs this change.
  • zig build test: 620 passed, 5 skipped, 0 failed. zig fmt --check: clean. lean_check: 538, as main.
• Not run, and why: a served Nemotron run on CUDA (no Nemotron checkpoint on these machines); Metal (CUDA only). weightShare, explains_open and the slide guard are not covered by a host test: they are branches of openWith, which needs a GPU and a real checkpoint to reach. The GLM-5.3-Flash family's runs at TP=2 and TP=3 (Zig CUDA ports: shared layers below lanes.Backend, and one place to coordinate them #548) go through all three.

Checklist

• Every token still goes through the lane rounds. No serial path, and nothing that needs drafts off.
• Drafted output equals "draft": false, a resumed prompt equals a fresh one, and concurrent equals solo.
• No precision traded for speed. If the bits change, the description says which sums change and why.
• Prompt processing is no slower than the last release.
• The description says which platforms I ran, Metal M1 to M5 and CUDA, and which I could not.
• New tests fail before the change, pass after it, and skip cleanly without their dependency.
• Comments and docstrings are one line. No measurements or history in the source.
• No personal data, machine names, internal hosts or local paths. No AI attribution lines.

[Bizuayeu]

Rebased onto 1.0.9. Its Nemotron store gained warm turns (.warm = .waits, warm_turns), so cuda_snaps.cacheRules now sets .waits for a family that declares warm_quiet (Nemotron does) and warm_turns follows the same declaration; Nemotron's rules, budget and warm turns are 1.0.9's, and a family without warm_quiet keeps none. The test covers the three cases. Host tests 430/430, zig build test 620 passed, 5 skipped.