fix(core/tree-sitter): render markdown backslash escapes as the escaped character

[3aKHP]

Fixes #1369

Summary

Markdown renderables style paragraph text through the markdown_inline injection with conceal. The bundled highlight query captures (backslash_escape) as @string.escape with no conceal rule, so CommonMark escapes rendered literally with the backslash visible and escape-styled (\~ displayed as backslash + tilde in the escape color).

The escape cannot be fixed in the query language: the markdown_inline grammar emits backslash_escape as one atomic regex token spanning both characters, so no query can capture the backslash separately, and conceal replaces whole captured ranges with a static literal, so a whole-node conceal rule would remove the escaped character too.

Fix

Both halves in the worker's one-shot highlight pipeline:

• processInjections spreads each injected capture node into a plain pseudo-node; web-tree-sitter's node type is a prototype getter, so the spread dropped it. Carry type explicitly so downstream consumers can dispatch on it.
• getSimpleHighlights splits a backslash_escape capture that has no query-configured conceal into a one-byte conceal of the backslash and drops the escape styling from the remainder, so \~ renders as plain ~. An explicit #set! conceal on the node set by a query still wins.

The marked-based streaming placeholder path already decoded escapes (renderInlineToken's escape case), so the two internal render paths now agree.

Tests

client.test.ts gains a markdown escape test asserting that each string.escape highlight on "Escaped \~ and \* here" collapses to a one-byte conceal tuple covering only the backslash ([8,9) / [15,16), meta.conceal === ""), and that no full-range string.escape remains, so the escaped character renders unstyled.

tree-sitter suites: 63 pass, 1 pre-existing skip. The only packages/core failure (node-assets manifest) also fails on the pristine tag without build output and is unrelated.

Out of scope (deliberately)

• (hard_line_break) keeps its current literal rendering; it shares the @string.escape capture but involves line-break semantics rather than character decoding.
• #offset!-style partial-token conceal (nvim queries use it in commented form) would be a more general engine feature; this fix does not need it.