codeSplitter line locks 2: wait in line, and soft intents #927

Design: apps/codesplitter/docs/line-locks.md (#925). Follows phase 1.

• claim_lines {wait: true}: FIFO per file, a socket grant with a 15 s grant window, and MCP/CLI long-polls capped at about 20 s (an open request keeps the DO awake, which bills duration)
• wait is refused to a caller holding claims in another file (no cross-file hold-and-wait, so no deadlock)
• soft: true intents ("rewriting 40–80"): shown and listed on the page, but edits aren't refused