#366 switched `ConcatSourceMapBuilder` to borrowed `&'a str` storage, which is
optimal when inputs outlive the result. But its main consumer (rolldown's
`SourceJoiner`) feeds *owned* `SourceMap<'static>` maps and wants an owned map
back, so `into_owned_sourcemap` deep-copied every name/source/sourcesContent
string — and `sourcesContent` is the full module text.

Store entries as `Cow<'a, str>` again so the builder can absorb owned strings by
moving them, and add:

* `add_sourcemap_owned(SourceMap<'a>, u32)` / `from_owned_sourcemaps(..)` — consume
  owned maps and move their `Cow` entries in (no string bytes copied).
* `add_sourcemap` (borrowed) and `add_sourcemap_owned` now share an `add_tokens`
  helper; only how the strings get in differs.

`into_sourcemap` moves the accumulated vectors straight out (also cheaper than
#366's rebuild for the borrowed case); `into_owned_sourcemap` keeps moved-in owned
strings as-is and copies only borrowed ones.

The `concat/owned_maps` benchmark (added in #369 as the borrow baseline) is switched
to the move-in path here. From the investigation sweep (200 modules): move-in is flat
at ~32us regardless of `sourcesContent` size, while borrow + `into_owned_sourcemap`
grows linearly (37us @256b, 47us @2KB, 70us @8kb, 542us @32kb) — up to ~16x faster,
identical mappings output.

#366 switched `ConcatSourceMapBuilder` to borrowed `&'a str` storage, which is
optimal when inputs outlive the result. But its main consumer (rolldown's
`SourceJoiner`) feeds *owned* `SourceMap<'static>` maps and wants an owned map
back, so `into_owned_sourcemap` deep-copied every name/source/sourcesContent
string — and `sourcesContent` is the full module text.

Store entries as `Cow<'a, str>` again so the builder can absorb owned strings by
moving them, and add:

* `add_sourcemap_owned(SourceMap<'a>, u32)` / `from_owned_sourcemaps(..)` — consume
  owned maps and move their `Cow` entries in (no string bytes copied).
* `add_sourcemap` (borrowed) and `add_sourcemap_owned` now share an `add_tokens`
  helper; only how the strings get in differs.

`into_sourcemap` moves the accumulated vectors straight out (also cheaper than
#366's rebuild for the borrowed case); `into_owned_sourcemap` keeps moved-in owned
strings as-is and copies only borrowed ones.

The `concat` benchmark (added in #369 as `ConcatSourceMapBuilder::from_sourcemaps`,
the borrow baseline) is switched here to `ConcatSourceMapBuilder::from_owned_sourcemaps`.
From the investigation sweep (200 modules): move-in is flat at ~32us regardless of
`sourcesContent` size, while borrow + `into_owned_sourcemap` grows linearly
(37us @256b, 47us @2KB, 70us @8kb, 542us @32kb) — up to ~16x faster, identical
mappings output.

#366 switched `ConcatSourceMapBuilder` to borrowed `&'a str` storage, which is
optimal when inputs outlive the result. But its main consumer (rolldown's
`SourceJoiner`) feeds *owned* `SourceMap<'static>` maps and wants an owned map
back, so `into_owned_sourcemap` deep-copied every name/source/sourcesContent
string — and `sourcesContent` is the full module text.

Store entries as `Cow<'a, str>` again so the builder can absorb owned strings by
moving them, and add:

* `add_sourcemap_owned(SourceMap<'a>, u32)` / `from_owned_sourcemaps(..)` — consume
  owned maps and move their `Cow` entries in (no string bytes copied).
* `add_sourcemap` (borrowed) and `add_sourcemap_owned` now share an `add_tokens`
  helper; only how the strings get in differs.

`into_sourcemap` moves the accumulated vectors straight out (also cheaper than
#366's rebuild for the borrowed case); `into_owned_sourcemap` keeps moved-in owned
strings as-is and copies only borrowed ones.

The `concat` benchmark group covers all four entry points: `from_sourcemaps` /
`add_sourcemap` (borrow — the baseline added in #369) plus `from_owned_sourcemaps` /
`add_sourcemap_owned` (the move-in variants added here). From the investigation
sweep (200 modules): move-in is flat at ~32us regardless of `sourcesContent` size,
while borrow + `into_owned_sourcemap` grows linearly (37us @256b, 47us @2KB,
70us @8kb, 542us @32kb) — up to ~16x faster, identical mappings output.

#370)

* Revert "bench: add a representative concat sourcemap benchmark (#369)"

This reverts commit 8d72a6a.

* Revert "refactor: apply borrow builder pattern to ConcatSourceMapBuilder (#366)"

This reverts commit e55562d.