feat: add OwnedSourceMap wrapper for downstream owned-only use

[Boshen]

Summary

Adds a thin newtype wrapper `OwnedSourceMap` around `SourceMap<'static>`. The wrapper hides the lifetime annotation that PR #329 introduced, giving downstream consumers (oxc, rolldown) a clean no-lifetime type for storing sourcemaps in long-lived struct fields.

Stacks on top of #329 (the borrow-string parse path). No allocation-layout change; this is purely an API-ergonomics layer.

```rust
use oxc_sourcemap::{OwnedSourceMap, SourceMap};

// Builder produces a no-lifetime map directly:
let owned: OwnedSourceMap = SourceMapBuilder::default().into_owned_sourcemap();

// Parse-and-detach in one step (uses #329's zero-copy parse path internally):
let owned = OwnedSourceMap::from_json_string(json)?;

// Same accessors as SourceMap, returning &str:
let _file: Option<&str> = owned.get_file();
let _names: Vec<&str> = owned.get_names().collect();

// Borrow back into the lifetime-parameterized API where needed
// (e.g. encode/concat/visualizer that are still generic over the lifetime):
owned.as_source_map().to_json_string();
```

What downstream looks like

Before (PR #329 alone):

```rust
pub struct Asset {
pub map: Option<oxc_sourcemap::SourceMap<'static>>,
// ^^^^^^^^^ noise
}
```

After (with this PR):

```rust
pub struct Asset {
pub map: Option<oxc_sourcemap::OwnedSourceMap>,
}
```

The `'static` annotation is gone from every storage site. Callers who genuinely want zero-copy parse still use `SourceMap<'a>` directly.

Downstream verified

• oxc: 2 small `-> OwnedSourceMap` annotations in `oxc_codegen`; the napi shim now accepts both `SourceMap<'_>` and `OwnedSourceMap`. `cargo check --workspace --all-targets` clean.
• rolldown: `rolldown_sourcemap` re-exports `OwnedSourceMap as SourceMap` so every existing `rolldown_sourcemap::SourceMap` usage upgrades for free. `adjust_sourcemap_dst_lines` and `collapse_sourcemaps` updated to the owned wrapper. `cargo check --workspace` clean, `rolldown_sourcemap` tests pass.

Breaking changes

None for the existing `SourceMap<'a>` API in #329 — this is purely additive.

Files

• `src/owned_sourcemap.rs` (new) — wrapper type, delegating accessors / parse / serialize / lookup.
• `src/lib.rs` — re-export `OwnedSourceMap`.
• `src/sourcemap_builder.rs` — adds `into_owned_sourcemap()` convenience.
• `src/napi.rs` — `impl From for napi::SourceMap`.

[codspeed-hq]

Merging this PR will improve performance by 6.07%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 9 improved benchmarks
❌ 3 regressed benchmarks
✅ 4 untouched benchmarks
⏩ 5 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	lookup_table[real_small]	1.4 µs	1.3 µs	+2.18%
⚡	add_sourcemap_loop	363 µs	343.2 µs	+5.76%
⚡	from_sourcemaps	344.3 µs	325.4 µs	+5.81%
⚡	lookup_table[real_medium]	1.5 µs	1.4 µs	+4.03%
⚡	parse[real_large]	54.7 µs	50.5 µs	+8.34%
⚡	parse[real_medium]	16.5 µs	14.9 µs	+10.72%
❌	serialize[real_small]	4.3 µs	4.4 µs	-2.44%
⚡	parse[real_small]	13.2 µs	11.6 µs	+13.16%
⚡	parse[real_xlarge]	1.7 ms	1.4 ms	+20.6%
⚡	from_json_string_inline	15.8 µs	14.1 µs	+11.97%
❌	serialize[real_large]	20.3 µs	20.7 µs	-1.9%
❌	serialize[real_medium]	4.9 µs	5.1 µs	-2.8%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing perf/owned-sourcemap (4ad61e8) with main (9323530)}

Footnotes

1. 5 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩