Integration: DSL grammar + parser support for rule annotations

[rrrodzilla]

Workstream: Integration · Part of #89

Context

Rule annotations need first-class DSL grammar + parser support, alongside the existing @hook/@access/field annotations (crates/schema-forge-dsl, crates/schema-forge-core/src/types/annotation.rs).

Scope

• Grammar + parser for @require, @compute, @default(expr: ...) (and any rules-file form).
• Parse errors with file:line diagnostics.
• Round-trip through schema (de)serialization.

Acceptance

• Rule annotations parse, with clear diagnostics on malformed expressions.

---

Dependencies: Co-requisite for #92/#93/#94; suggested order: early, alongside #91. Final annotation shapes are soft-informed by #91. Soft.

[rrrodzilla]

Done in commit 64e00d6 (branch `dev`).

The CEL rule annotations now have first-class DSL grammar + parser support, with `file:line:col` diagnostics on malformed expressions and full schema round-trip. Syntax (positional, CEL as a double-quoted string — chosen over named/triple-quoted):

```
age: Integer @require("age >= 18", "must be 18 or older")
name: Text @compute("first + ' ' + last")
ts: DateTime @default("now()")
```

• core: `FieldAnnotation::{Require { expr, message }, Compute { expr }, Default { expr }}` (serde + Display round-trip; core stays CEL-free, storing only the raw expression source). The expression `@default("...")` is distinct from the literal `default(...)` field modifier; both coexist.
• dsl: parser validates each expression syntactically via `schema_forge_cel::parse`; on error the intra-expression position is mapped through the DSL string escapes to an absolute `line:column`, so the diagnostic points into the expression (new `DslError::InvalidCelExpression`). Adds `schema-forge-cel` as a path dep (no cycle).

Scope: grammar + parser + round-trip + syntactic diagnostics only. Runtime enforcement is #92/#93/#94; apply-time type-checking against field types is #104. 628 tests pass, clippy clean.

This unblocks #92 (`@require`), #93 (`@compute`), #94 (`@default`).