add tree-sitter syntax highlighting via dart:ffi

Vendor libtree-sitter.so (v0.26.8, wasmtime 44.0 statically linked)
and call its C API through dart:ffi. Grammar WASM files are loaded by
tree-sitter's embedded wasmtime engine via ts_wasm_store_load_language.

48 grammars compiled with tree-sitter build --wasm, 48 highlight
queries (42 from upstream repos, 6 written in-house for comment,
erlang, gdscript, gitignore, hcl, hlsl, swift). SQLite grammar
replaces full SQL (974K-line parser exhausted RAM at compile time).

Removes wasm_run and wasm_run_flutter — they downloaded a 22 MB
binary from GitHub at first launch, violating the no-network-on-
default-launch-path policy.

Co-Authored-By: Claude <noreply@anthropic.com>

app/assets/queries/diff.scm

@@ -0,0 +1,49 @@
+(comment) @comment @spell
+
+[
+  (addition)
+  (new_file)
+] @diff.plus
+
+[
+  (deletion)
+  (old_file)
+] @diff.minus
+
+(commit) @constant
+
+(location) @attribute
+
+(command
+  "diff" @function
+  (argument) @variable.parameter)
+
+(filename) @string.special.path
+
+(mode) @number
+
+([
+  ".."
+  "+"
+  "++"
+  "+++"
+  "++++"
+  "-"
+  "--"
+  "---"
+  "----"
+] @punctuation.special
+  (#set! priority 95))
+
+[
+  (binary_change)
+  (similarity)
+  (file_change)
+] @label
+
+(index
+  "index" @keyword)
+
+(similarity
+  (score) @number
+  "%" @number)