fix(conformance): carry the schema the suite judges against

[glatinone]

fix(conformance): carry the schema the suite judges against

The default spec path was Path("spec/v0.1.0/memory-cell.schema.json"), relative
to the working directory. Run the suite the way it is meant to be run - against
somebody else's server, from wherever they happen to be standing:

amp-conformance --base-url https://their-server.example.com
FAIL schema: schema-spec-file -- spec not found at spec\v0.1.0\...; pass --spec

That is the suite failing to find its own fixture, reported in the same red as a
server failing a vector. A third party reading it sees a failure about a file they
have never heard of, naming neither their server nor their mistake. The goal of
this suite is to be runnable by implementations that are not this one, and a
directory-dependent fixture works against exactly that.

• The normative schema now ships inside the wheel (a hatch force-include from
  spec/v0.1.0/, so the source of truth stays in one place and the copy is
  generated, not duplicated in the tree).
• The default is resolved by searching the working directory, then each of its
  parents, then the packaged copy. Running from a checkout subdirectory now works
  too - that case was silently failing as well.
• When no schema can be found, the schema vectors report skip with the flag to
  pass, and the summary counts them separately. --only schema exits non-zero,
  because asking for those vectors by name and getting a skip that exits 0 is the
  exact failure mode this suite exists to catch in other people's servers.
• --spec is unchanged and still wins when given.

Two existing tests pinned the old behaviour; both are rewritten to assert the new
one rather than deleted - one of them existed precisely to check that a missing
spec was visible, and that requirement survives as "a skip is counted and named".

Verified: 19 conformance tests pass, the suite run from conformance/ now reports
14 schema vectors passing where it previously reported a failure, and the built
wheel contains amp_conformance/spec/memory-cell.schema.json.

[glatinone]

Landed on master in the v0.1.0 chain: the branch was fast-forward merged as part of b940905..92b88ee and released as v0.1.0. Closing so the open list matches reality - the commits are in master, and the tag points at them.