scheme: recursive_block_lisp has no slicing mode of its own, falls through to Mode_B_Braces; start_line wrong beyond the first declaration

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Found while building #2908 Phase 2 contract tests (not introduced there; present on main).

recursive_block_lisp has no ScopeParsingRegistry entry and appears in no Mode-A/Mode-C special-case tuple in _function_slice, so scheme falls through to the generic else at detector.py:3173 (mode_name = "Mode_B_Braces") — brace slicing for a language whose delimiters are parens. Confirmed: a single (define (f) 1) preceded only by blank/comment lines always reports start_idx=0 / start_line=1 regardless of true position, because Mode B has no real brace boundary to anchor on.

Consequences: per-function start_line/loc are unreliable for scheme on multi-declaration files, and #2908's header-window is_documented anchor inherits the wrong start_line. The Phase 2 contract tests route around it (single-declaration snippets only).

Fixing this moves existing extraction counts, so it is its own rule/slicer-layer PR per #2812 rule 4.