Internal: Dogfooding cleanup, phase 4

Continues v1.18.0's pass: hardcoded thresholds and keyword lists become plugin config; cha fix stops hardcoding smell names; the last two text-scanning detectors switch to AST queries.

Added

• Plugin::try_fix(finding, ctx) -> Option<Patch> — every plugin can now contribute auto-fixes. cha fix walks all enabled plugins and asks each one. Adding fix support for a new smell is one trait override, not a host-side if smell_name == ... patch.
• cha_core::Patch / cha_core::TextEdit — public byte-range edit types for plugin authors. Edits within a single finding are applied in reverse byte-offset order.
• DesignPatternAdvisor config — 8 magic-number thresholds (strategy_min_arms, state_min_arms, builder_min_params, builder_alt_min_params, builder_alt_min_optional, null_object_min_count, template_min_self_calls, template_min_methods) and 2 keyword lists (type_field_keywords, state_field_keywords) are now overridable via [plugins.design_pattern].
• GodClassAnalyzer::min_tcc — Tight Class Cohesion threshold (Lanza-Marinescu's 1/3) is configurable.
• cache.rs walker skip-dirs — extended from {target, node_modules, dist} to also skip build, out, __pycache__, venv, .venv, vendor. Reduces unnecessary .cha.toml discovery work in polyglot repos.

Fixed

• switch_statement / message_chain: replaced bespoke text scanners (find_switch_keyword, walk_chain and ~140 lines of hand-rolled tokenization) with tree-sitter queries against switch_statement / match_expression / field_expression / member_expression / selector_expression. lvgl baseline counts unchanged (139 / 61), but keyword positions are now sourced from the AST. No more false positives on keywords inside strings.
• inappropriate_intimacy import resolution: extension probe expanded from {.ts, .tsx, .rs} to also include .py, .go, .cpp, .cc, .cxx, .c, .h, .hpp, .hxx, .js, .jsx, .mts, .cts. Sibling-file lookups in non-JS/Rust projects no longer silently fail.
• calibrate.rs table rendering: 3 metric labels were hardcoded across 3 separate println blocks. Adding a calibration metric now means one entry in a (label, samples) array.

Changed

• cha fix delegates to Plugin::try_fix for every finding — no more filter(|f| f.smell_name == "naming_convention") on the host. Existing behavior preserved (NamingAnalyzer fixes naming_convention PascalCase violations); other plugins return None until they opt in.