grammar: open subsystem keys to arbitrary identifiers (closes #64)

[proofmancer]

Closes #64.

The chain manifest grammar was hardcoded to five subsystem keys (consensus, gas, state, exec, da). A chain wanting to declare a new axis (privacy, MEV protection, cross-chain bridging, anything we did not anticipate) could not. This PR lifts that.

What changed

Parser

`is_subsystem_key` now accepts `TK_IDENT` in addition to the five reserved keyword tokens. Any identifier is a valid subsystem key. The five names stay as keyword tokens because `state` and `gas` also start module-item declarations elsewhere in the grammar.

Grammar spec

`spec/grammar.ebnf`: `SubsystemKey = IDENT` instead of the enumerated list. The five names are documented as the stdlib axes, but no longer parser-level requirements.

Tree-sitter

`cleave-lang/tree-sitter-cleave` updated in the same change so the two grammars stay in lockstep. `subsystem_key` becomes `$.identifier` instead of a fixed-choice rule. Test corpus fixtures updated to expect the new `(subsystem_key (identifier))` shape.

Tests

`test_error_unknown_subsystem_key` deleted. It asserted that `unknown: ...` errors; after this PR that's the intended behavior. Replaced with two positive tests:

• `test_arbitrary_subsystem_key_parses`: `chain C { privacy: GrothProver<curve=BN254> }` parses, key is "privacy"
• `test_subsystem_key_can_still_be_stdlib_keyword`: regression guard that `consensus:` and friends still parse the same way

154 parser assertions (was 149).

End-to-end demo

```cleave
// This parses now (previously rejected)
chain MyZkChain {
consensus: Tendermint<sig=BLS12_381>
gas: Multidim<{cpu, storage, witness}>
state: SparseMerkle<key=u256, hash=Blake3>
exec: WasmVM
da: NativeDA
privacy: GrothProver<curve=BN254>
mev: FlashbotsBundler
bridge: CelestiaIBC
}
```

What this PR does NOT do

• No protocol-satisfaction check: the type checker does not verify that `GrothProver` actually implements anything. That's the typecheck-extension work in Type checker: validate AST before codegen #34 ("trait/protocol satisfaction" is explicitly out of scope today) and RFC: third-party protocol extension story (publish, discover, audit, version) #65 (the third-party protocol RFC).
• No packaging / discovery: how does someone publish `GrothProver`? How does another chain depend on it? Open in RFC: project metadata format + build system for Cleave projects #45 (project metadata RFC) and RFC: third-party protocol extension story (publish, discover, audit, version) #65 (third-party protocols RFC).
• No runtime support: the runtime still only handles the WASM + EVM execution subsystems. New subsystem keys parse but have no runtime hook.

This PR is just the parser-layer lift. The full third-party extensibility story needs the three follow-ups above.

What unblocks next

• RFC RFC: third-party protocol extension story (publish, discover, audit, version) #65 (third-party protocol extension) can now reference concrete syntax that already works
• Authors experimenting with custom protocol designs can write proposals against a manifest grammar that does not need to be patched