fix(viewer): harden conversation record inspection

[siriusctrl]

Summary

This is an independent review/fix layer on top of the conversation JSON implementation in #7.

• require explicit media context before collapsing sibling payloads, and validate standard versus URL-safe base64 alphabets separately
• make nested typed tool results suppress duplicate envelope links while preserving explicit canonical fallbacks
• retain exact lazy JSONL records in an immutable raw spool so source rewrites cannot change an open snapshot
• force a full terminal redraw when switching between structured and raw content spaces
• bound invalid UTF-8 boundary adjustment to at most three bytes

Root cause and impact

The original display paths mixed a few independent assumptions: payload characters could imply encoding, a role-tool envelope could compete with its typed child, lazy raw mode reread mutable source bytes, and terminal scroll deltas crossed content-space boundaries. Together those could produce false media summaries, incorrect tool navigation, non-repeatable raw records, or stale terminal rows after r toggles.

The fix keeps redirected output and tool arguments token-preserving, keeps media scanning streaming, and leaves all terminal lifecycle/rendering ownership outside fmtview-core.

Validation

• focused adversarial suites: media transform 27, tool links 24, lazy/raw 4, headless viewer 8
• cargo test --workspace --all-targets --no-fail-fast
• cargo clippy --workspace --all-targets -- -D warnings
• RUSTDOCFLAGS=-Dwarnings cargo doc --workspace --no-deps
• cargo publish -p fmtview-core --dry-run --locked
• 16 MiB media load smoke: 48.023 ms median, formatted spool held to 58 bytes
• fresh Kitty/Xvfb flow plus a long-record r -> j x48 -> r recording; raw moved from 20% to 53% and the structured screen restored cleanly
• final PTY capture: alternate screen and all mouse modes balanced 1:1, synchronized updates balanced 5:5, exact raw/structured screen markers passed

Local durable evidence is under target/fmtview-emulator-recordings/raw-scroll-review-d80e725-3/: Kitty video/contact sheet and text snapshots, plus pty.log, pty-check.json, pty-raw-screen.txt, and pty-structured-screen.txt.

Documented streaming limitations

• sibling metadata must precede data for streaming recognition
• escaped data-URI headers remain uncollapsed

Raw search also intentionally does not match across the artificial 32 KiB chunk seam.