`ConcatSourceMapBuilder::add_sourcemap` stores `Cow::Borrowed` views into each
input map, so a concatenated result borrows from its inputs. A consumer that
needs an owned (`'static`) result — e.g. rolldown joining per-module maps into
one chunk map — must then call `into_owned`, deep-copying every name / source /
`sourcesContent` string: one allocation per string, on a hot per-chunk path.

Add owned-merge entry points that take a `SourceMap` by value and **move** its
`names` / `sources` / `source_contents` storage straight in via
`SourceMap::into_parts` — no string bytes copied. The merged map is then already
owned, so the consumer can drop `into_owned` entirely:

* `add_sourcemap_owned(SourceMap<'a>, u32)` — owned counterpart of `add_sourcemap`.
* `from_owned_sourcemaps(..)` — owned counterpart of `from_sourcemaps`.
* `add_sourcemap` (borrowed) and `add_sourcemap_owned` (owned) share a private
  `add_tokens` helper, so the translation / boundary-dedup / `TokenChunk`
  bookkeeping can't drift between the two paths.
* `into_owned_sourcemap` is kept for callers wanting a `'static` result from
  borrowed inputs.

`add_tokens` translates ids on their raw representation (`Token::translated`, no
`Option` round-trip), bulk-`memcpy`s the first map when it needs no offset,
appends the rest via `extend`, and derives the next chunk's prev-id baseline with
a single backward scan — keeping the translate loop branchless so it
auto-vectorizes. The memcpy fast path is gated on all offsets being zero, so a
prior map contributing sources/names but no tokens still renumbers correctly.

Both paths pad `source_contents` with `None` to keep it index-aligned with
`sources` (input `sourcesContent` may be absent or shorter than `sources`).

Existing signatures are untouched. Tests assert the owned path produces
byte-identical output to the borrowed path, plus the two edge cases above.