Annotate jawohl itself: no surface crate, and Stream crosses as a handle

[zmaril]

Replaces the hand-written surface crate with annotations on jawohl's own
definitions, and upgrades the bindings from a batch shim to the real
incremental parser.

What changed

bindings/surface is deleted. It was a crate whose whole job was to restate
jawohl's API: two enums mirrored variant for variant with From impls in both
directions, plus eight functions wrapping Stream. Nothing connected it to the
thing it described, so nothing could catch it drifting.

The annotations now sit on the definitions:

• Syntax and Validation derive jedem::Enum where they are declared
• complete_json and get_closing_string_for_partial_json carry #[jedem::export]
• the Stream impl carries it once, for the whole block

There is no hand-written code under bindings/ any more — only this README and
the two host tests.

Stream is a handle now

Python and JavaScript get the incremental parser itself. The old shim re-parsed
from byte zero on every call, which is quadratic over a document — the exact
thing the library exists to avoid.

s = jawohl.Stream.from_json_schema(schema)
for chunk in response:
    s.push(chunk)
    if s.is_irrecoverable():
        break                      # stop paying for a generation that cannot succeed

The host tests are rewritten around it, and now assert something the batch API
could not express: cancellation lands on the chunk that decided the schema was
unsatisfiable, not at end of document.

What is still not across

The two Rust move-builders cross unchanged too — with_number_profile and
with_max_depth take self and return Self, which names the same object, so
jedem mutates in place and hands the same handle back. Stream().with_max_depth(64)
chains in Python and JavaScript exactly as in Rust, with no annotation on either
method.

Four methods carry #[jedem(skip)], each with its reason in place:

method	why
snapshot, changes	need unions — Value is recursive, Event has struct variants
error, lowering_report	need records, and return a reference

changes is the one to want back: it is how events would be consumed instead of
polling status.

Two consequences worth stating plainly

jawohl now depends on jedem. Nothing survives to runtime — the descriptor is
&'static data, and the generator is reached only from the #[cfg(test)] drift
guard — but it is a real dependency on an unpublished crate. cargo package
cannot resolve it, so jawohl 0.2.0 cannot ship until jedem 0.1 is on
crates.io. The packaging job is marked continue-on-error with that written
beside it, so the failure stays visible without turning unrelated PRs red.

The MSRV rises from 1.70 to 1.78. jedem uses
#[diagnostic::on_unimplemented], stabilised in 1.78, and its proc-macro chain
floors at 1.71. Declared rather than patched around, and verified against a real
1.78.0 toolchain. jedem now declares the same floor in its own manifest, so the
next crate to depend on it does not have to discover this the way jawohl did.

The dependency currently points at a jedem branch (feat/handles), because
the handle and bindings: support it needs is in jedem PRs #11 and #12. Those
merge first; then the branch key comes off.

The drift guard moved into jawohl's own test suite, since the surface is now
jawohl's annotations. cargo test checks the committed bindings, JEDEM_WRITE=1 cargo test rewrites them.

Verified

• cargo test --all-targets, cargo test --doc, clippy and fmt all clean
• both host processes pass end to end against freshly generated bindings