feat(types): carry the (echo, epi) grade in function types (ADR-0009 D1/D3)

[hyperpolymath]

The second half of "type annotate and @epi" — carries the (echo, epi) effect row in a function type, with the prefix-marker syntax you chose. With this, the ADR-0009 D1+D2+D3 declaration + type effect surface is complete.

fn apply(f: @echo(Neutral) @epi(Partial) Fn(Int) -> Int): Int { ... }

What changed

• ast.rs: EffectGrade { echo: Option<Echo>, epi: Option<Epistemic> }; TypeAnnotation::Function gains a trailing EffectGrade; Type::Function likewise.
• grammar/parser: function_type accepts leading @echo/@epi markers (same tokens as on declarations); the parser splits them from the param/return types.
• annotation_to_type carries the grade into Type::Function; Display + pretty + formatter emit the prefix markers (round-trip).
• coercible_to: a function type is covariant in the grade (grade_leq) — a function that loses/reveals less is usable where more is allowed; a None target is unconstrained; a None source can't satisfy a Some ceiling; params/return invariant for now.

Honest scope (as discussed)

This is representational / forward infrastructure: JtV has no first-class function values yet (no lambdas / indirect calls), so the value-against-type coercion is correct but latent until function values land. It completes ADR-0009's stated surface and is the right shape for when they do.

Tests / quality

4 new tests (parse graded + ungraded; round-trip; covariance accept/reject/unconstrained). 142 lib tests, cargo fmt + clippy clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01BJmfoz1ZS1Pejy9LLMY742

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Generated by Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 55 issues detected

Severity	Count
🔴 Critical	0
🟠 High	23
🟡 Medium	32

View findings

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