chore(deps): sharp 0.35.5 and a Node 20.9 floor

[keiver]

sharp 0.33.5 carries open advisories in its bundled libvips, libheif and librsvg. 0.35.5 is the first release with all three fixed.

Changes

• sharp ^0.33.5 to ^0.35.5.
• Node floor 18 to 20.9, which sharp 0.35.5 requires: engines, src/check-node-version.ts, the README, and node-version in both workflows.
• sharp 0.35's ESM typings have no sharp namespace: sharp.Sharp and sharp.OverlayOptions become named type imports (5 sites), and one let buffer is annotated Buffer.

Measured

• npm run build clean; npm test 30 suites, 416 tests pass.
• Default run and the full examples/tomotv config generated on 0.33.5 and on 0.35.5: same 101 files, same dimensions and channel counts.
• 35 of 52 PNGs are pixel-identical. 15 differ in at most 123 pixels by 1 level of 255. The two derived tinted icons shift by about 2 grey levels (187 to 189) from grayscale().

Raising the Node floor is a breaking change for consumers on Node 18. No version label is set.