> [!NOTE]
> Draft. Builds on the merged diagnostics cleanup (#24661), and has been through two `/simplify` review passes.
>
> One design point a perf reviewer may ask about: `ParserCheckpoint` (copied on every `checkpoint`/`rewind`/`lookahead`) embeds `FatalError` **inline**, not boxed. Boxing it would shrink the checkpoint from 3 → 2 cache lines, but only by adding a heap allocation — counter to this PR's goal. The deferral already shrank the checkpoint substantially for free: `FatalError` previously held an inline `OxcDiagnostic` (~150 B), now a 72 B POD `ParserDiagnostic`.

Parser diagnostics are now stored in a deferred (unmaterialized) form during parsing and materialized into `OxcDiagnostic` only once, at parse exit.

## Design

A `parser_diagnostics!` macro in `diagnostics.rs` generates, from one entry per diagnostic:
- an enum variant `ParserDiagnostic::Foo { .. }` holding only the POD arguments (`Span`, `&'a str`, `Copy` types);
- a `#[cold] into_diagnostic()` match arm holding the eager construction (box + label strings + `format!`) — all the allocation/formatting work concentrated in one cold function;
- an `#[inline]` constructor with the **same name and signature as before**, so the ~330 `diagnostics::foo(...)` call sites are unchanged.

The parser/lexer error vecs and `FatalError` now carry `ParserDiagnostic<'a>`, so pushing an error, snapshotting the error set on `checkpoint()`, and discarding it on `rewind()` never allocate. Materialization happens at the two `parse()` / `parse_expression()` exit points. Foreign already-materialized diagnostics (from `oxc_regular_expression`) ride an `Eager(Box<OxcDiagnostic>)` escape hatch; the two module-record diagnostics that are built once at exit stay eager.

## Why

This eliminates the eager boxing of diagnostics that are created and then thrown away during speculative parsing (arrow-function / type-vs-expression backtracking). On real-world files — all valid, so nearly every one of these allocations was a speculation-discarded diagnostic:

| file | sys allocs before → after |
|---|---|
| checker.ts | 1818 → 23 |
| antd.js | 3661 → 190 |
| kitchen-sink.tsx | 2051 → 190 |
| App.tsx | 360 → 23 |
| binder.ts | 85 → 1 |

Diagnostic output is byte-identical — parser conformance (Test262, Babel, TypeScript) snapshots are unchanged.

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