fix(strategy): declare frac in every contributor-facing basis() snippet (Fixes #274)

[luciferlive112116]

Fixes #274. Re-lands #253 (rolled back with its whole merge window) and extends it
to the copy #253 missed.

PR kind

• fix
• feat

Summary

multiply_subspace forwards the VRAM budget only to a basis() that declares
frac — and its else is a compatibility shim for legacy transforms:

# Pass frac only if the transform accepts it, so custom transforms with the
# older basis(...) signature keep working.
if "frac" in inspect.signature(transform.basis).parameters: ... frac=frac
else:                                                        ... # no frac

So any snippet a contributor copies that omits frac yields a transform that lands
in the legacy branch and silently streams its basis at _DEFAULT_ROW_BLOCK_FRACTION
(0.3) instead of Config.vram_fraction — the bug #211 fixed for rsvd,
re-introduced once per contribution.

Three copies contributors read are currently on the pre-#211 signature:

copy	before
CONTRIBUTING.md:73 — "What you actually change"	❌
strategy/README.md:54 — "Register your own (the updatable hook)"	❌ never covered by #253
strategy/examples/run_example.py:45 — FirstAxes	❌

The authoritative surfaces already declare it: Transform.basis, transforms.py's
module-docstring template, rsvd (#211), and examples/transform_template.py
(#251 — still on main).

Why two of them regressed

#253 fixed CONTRIBUTING + run_example, but was rolled back with the entire
9058d49..f35482a merge window: 1dc8fce..e42c525 reverted all seven PRs of
that batch, LIFO, across five authors (#235, #232, #253, #266, #267, #268, #255).
This re-lands that content rebased on current main. strategy/README.md's hook
was never covered by #253 and is genuinely new here — it's the copy sitting
directly under "The transform is the pluggable core tech", so the most likely one
to be copied.

Changes

• CONTRIBUTING.md, strategy/README.md, strategy/examples/run_example.py —
  declare frac=None and say what it's for. FirstAxes is a fixed basis that
  streams nothing, so it notes the parameter is unused there but must stay, since
  the forwarding gate is signature-based. +13/−5.
• tests/test_contributor_docs_basis_signature.py (new) — CPU-only: regex-checks
  both markdown snippets and AST-parses the example (it can't be imported —
  it runs a real subspace_matmul at module scope, which needs a GPU), asserting
  each declares frac and that the example matches Transform.basis exactly.

No shipped engine code changed.

Validation

$ python -m pytest tests/test_contributor_docs_basis_signature.py -v
  3 passed

$ python -m pytest tests/ strategy/tests/ eval/tests/ -q     # CI-equivalent (no torch → GPU tests skip)
  380 passed, 24 skipped

No protected paths touched — root CONTRIBUTING.md, strategy/README.md and
strategy/examples/ are outside eval/, docs/, .github/, dashboard/.

Result

Contributor-facing docs/example + parsing tests; no shipped code path changed, so no
accuracy / latency / VRAM delta (it stops future transforms from silently losing
--vram-fraction in their basis stage).

metric	value
accuracy	unchanged (docs/example + tests only)
time complexity	unchanged
latency	N/A
VRAM usage	N/A here; prevents copied transforms ignoring vram_fraction

[github-actions]

This PR currently has merge conflicts with the base branch. Please resolve them within 12 hours or the bot will close the PR automatically.

[github-actions]

Closing this PR because the reported merge conflict was not resolved within 12 hours of the bot reminder.