build(deps-dev): bump rolldown to ^1.2.1 and unpin

[lukecotter]

rolldown 1.1.5 → ^1.2.1, and the exact pin becomes a caret range.

The pin dated from the pre-1.0 period. Now that v1 is stable there is no reason to hold
rolldown at an exact version while every other build dependency tracks a caret.

Kept out of the grouped dev-dependency PR (#880) on purpose: rolldown is a build tool
mid-consolidation, so a regression here needs to be revertable on its own.

Breaking changes

None documented. experimental.lazyBarrel — the one behaviour change worth checking — was
already defaulted off in 1.1.4. @rolldown/plugin-node-polyfills is already ^1.0.3 and
needs no change.

Verification

All four build variants built clean (rollup × rolldown, minified × unminified). The
rollup outputs are byte-identical, confirming the change is contained to the rolldown path;
the rolldown outputs shrink marginally:

Variant	Before	After	Delta
rollup unmin	18,266,842	18,266,842	+0
rollup min	8,836,315	8,836,315	+0
rolldown unmin	15,533,924	15,530,264	−3,660 (−0.024%)
rolldown min	9,036,480	9,031,430	−5,050 (−0.056%)

Totals are the summed .js + .css bytes across lana/out and log-viewer/out. Per-chunk
the only growth is the rolldown runtime shim (+38 bytes unmin / +22 min); everything else is
flat or smaller.

• pnpm lint ✅
• pnpm test ✅ 80 suites / 1186 tests

New capabilities, not adopted here

1.2.x adds dev-mode HMR with per-client patch maps, a hotUpdate hook, and
full-reload-on-tsconfig-change. That is a genuine candidate for the pnpm watch webview
loop, but it is feature work rather than a dependency bump — noted as a follow-up.

Also new and currently unused: the resolveFileUrl hook,
import.meta.ROLLDOWN_FILE_URL_*, sourcemapFileNames, and --configLoader=native.