feat(runtime): give ArchiveRead search, line reads, and decoded paging

[liugddx]

What

Once a large tool result is archived, retrieving content from it was awkward (issue #4355, part of #4267):

• no search-within-archive — you could only page blindly by character offset;
• character-only reads — poor for line-oriented terminal output;
• read sliced the JSON-escaped serialization — pages landed on escape bytes (\n, \") rather than decoded text;
• inspect gave no preview or coordinate space for plain-text / object payloads (only keys for objects, valueType for scalars).

This extends ArchiveRead within the existing ≤TOOL_RESULT_ARCHIVE_MAX_RESPONSE_CHARS bounded-response invariant, so a read can never itself trigger another archive:

• search operation — a literal, case-insensitive substring scan returning matched offset, line, and a bounded context snippet per hit; capped per page (TOOL_RESULT_ARCHIVE_MAX_SEARCH_MATCHES) with nextOffset/hasMore to resume.
• line-based reads — unit: "line" pages whole lines by line offset/limit, trimmed by binary search to the response budget, with nextLineOffset. unit defaults to "char" so existing callers are unaffected.
• decoded paging — string (terminal/text) payloads page the decoded string so offsets land on real characters; structured payloads still page their canonical JSON.
• inspect positional index + preview — text payloads report totalChars/totalLines and a short preview; object/scalar payloads report totalChars and a preview (objects with an items[] manifest keep their existing shape).

Why it's safe

• Search is literal, never a regex — no pathological backtracking, no ReDoS.
• Every response path (search, line, char, inspect) is bounded to TOOL_RESULT_ARCHIVE_MAX_RESPONSE_CHARS — search stops adding matches, and line reads binary-search their line count, before the envelope overflows.
• Scope is retrieval ergonomics only: the archive threshold behavior (active-tool-result-prune.ts) and the upstream Shell capture/preservation contract are deliberately untouched, per the audit.

Tests

Six regression tests in tool-result-archive-resource.test.ts:

1. inspect surfaces a positional index + preview for text payloads.
2. read pages the decoded string, not the escaped JSON.
3. read by line range returns whole lines and resumes with nextLineOffset.
4. search locates a case-insensitive substring with offsets, line numbers, and snippets.
5. search stays within the response budget and resumes (nextOffset) under a pathological dense-match input.
6. search without a pattern fails closed.

Causal proof (fail-without / pass-with): with the production dispatch temporarily reverted to the old char-only / serialized-JSON behavior, all six new tests fail (# fail 8, the extra two being the pre-existing inspect-manifest tests under the crude stub); with the change restored, the suite is green (# pass 11).

Part of #4267. Closes #4355.