pgw#1143 steps 1-3 (§1.33): a slot DECLARES the tensor layouts its code can execute

[PaulFidika]

The code side of the layout contract had no declaration surface at all —
api/slot.py carried pipeline_cls, selected_by, family, default_checkpoint, root, optional and nothing else, so the hub's th#1803 rebind gate was keyed
off the wheel's decoder census (2 distinct signatures across 64 releases)
rather than off what the author says the slot needs.

Slot(layouts={component_path: (handle, ...)}) is that declaration: an
ordered tuple per component path, "*" for the whole tree, order IS
preference (§1.33 pt 2). It is published into the release manifest as
functions[].slots[].layouts for the hub to gate against (th#1809 T1-T2).

• Absent is UNDECLARED, a tri-state — never "accepts everything" and
  never "accepts nothing". An empty mapping and an empty tuple are both
  decoration-time errors; collapsing the tri-state is th#1580's fail-open
  defect wearing a new name.
• On Slot, not Compile. Compile's fields feed contract_axes(),
  a cell-key input, and §1.33 pt 5 is that conversion is upstream of compute
  and invisible to cell identity. A layout declaration there would re-key
  every cell in the fleet for the one fact that must never enter the key.
• Handles at the constructor, KEYS at decoration. The handle vocabulary
  is KNOWN_CONTRACTS (refusal text reused verbatim from
  tensor_layout_contract._validate); the component keys are checked
  against the DERIVED component tree in _spec_for_handler, because a key
  that matches nothing reads as a declaration and gates nothing.
• The SDK emits HANDLES, never digests (th#1580 A2: descriptors are Go).
  The hub resolves handle -> Contract.Digest() at manifest ingest against
  its own registry — the only moment one wheel and one hub are both pinned —
  so KNOWN_CONTRACTS is honestly a transcription that may be stale and is
  CHECKED. hf.fp8-blockwise@1 joins it: registered hub-side since th#1803
  (226070fd), and its absence made §1.33's own worked example (an fp8
  text_encoder against a bf16 DiT) undeclarable.
• A composite <topology>+<quant> handle is refused, naming th#1809 T3.
  The topology axis has no registry to compare against yet; storing half a
  pair is not exact.
• The census stays a LOWER bound. A declared handle no
  @implements_contract decoder backs lands on the manifest as
  layouts_census_unbacked and does NOT refuse — the blockwise conditioner
  is decoded natively by transformers through quantization_config with
  zero cozy markers, and refusing would make the motivating case illegal.
• scripts/lint_layout_declarations.py (new fast gates step) refuses a
  declaration the AST sweep cannot read. Discovery imports endpoint modules
  torch-stubbed, and pgw#1107's sweep reads them without importing at all; a
  computed declaration is legible to neither, which would make the published
  manifest the only place the demand can be read.

RED on master, measured: Slot(P, layouts=...) -> TypeError: unexpected keyword argument 'layouts'; _slot_to_manifest emits
['components', 'name', 'pipeline_class']; KNOWN_CONTRACTS holds 5 of the
hub's 6 handles.

Pairs with tensorhub th#1809 T1-T2 (the hub-side gate that reads this). Steps 1-3 are additive: no endpoint declares layouts= yet, so every existing manifest is byte-identical and every slot stays UNDECLARED until an author opts in. No release cut — this joins the next batch.

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