feat(lsp): n6 BOM diagnostic + parity test fixtures (#5)

Close the PoC fidelity gap: validate_n6 now emits "byte-canonical: UTF-8 with
no BOM" via byte_at (ef bb bf), avoiding the encoding-fragile U+FEFF source
literal. Verified end-to-end (LSP didOpen of a BOM doc → publishDiagnostics
carries the BOM diag). Adds lsp/test_fixtures/ (9 cases: BOM/CRLF/indent-header/
unknown-type/malformed/continuation-indent/unrecognised/comments/valid) as a
.py-parity regression reference (run: python3 lsp/n6_lsp.py --check <fixture>).

Co-authored-by: dancinlife <dancinlab@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people

lsp/n6_lsp.hexa

@@ -180,7 +180,11 @@ fn first_token(t: string) -> string {
 // any generic stdlib validator (commons g61 advisory).
 fn validate_n6(text: string) -> string {
     let mut diags = []
-    // BOM check skipped (encoding-fragile in hexa) — see PR body fidelity gap.
+    // BOM: UTF-8 byte-order-mark ef bb bf at offset 0. byte_at avoids the
+    // encoding-fragile U+FEFF source literal — mirrors .py text.startswith(BOM).
+    if len(text) >= 3 && byte_at(text, 0) == 239 && byte_at(text, 1) == 187 && byte_at(text, 2) == 191 {
+        diags.push(diag(0, 0, 1, "byte-canonical: UTF-8 with no BOM", 1))
+    }
     if text.contains("\r") {
         diags.push(diag(0, 0, 1, "byte-canonical: LF line endings only", 1))
     }

lsp/test_fixtures/f1_valid.n6

@@ -0,0 +1,4 @@
+@P c_light = 299792458 :: physics [tier=0 active]
+  -> derived
+  "speed of light"
+  unit = m_per_s

lsp/test_fixtures/f2_unknown_type.n6

@@ -0,0 +1 @@
+@Z foo :: dom [g]

lsp/test_fixtures/f3_indent_header.n6

@@ -0,0 +1 @@
+  @P foo :: dom [g]

lsp/test_fixtures/f4_malformed.n6

@@ -0,0 +1 @@
+@P justtext no header

lsp/test_fixtures/f5_cont_indent.n6

@@ -0,0 +1,2 @@
+@P x :: d [g]
+    -> toomuch

lsp/test_fixtures/f6_cont_unrec.n6

@@ -0,0 +1,2 @@
+@P x :: d [g]
+  blah blah no edge

lsp/test_fixtures/f7_comments.n6

@@ -0,0 +1,3 @@
+# a comment
+
+# another

lsp/test_fixtures/f8_crlf.n6

@@ -0,0 +1,2 @@
+@P x :: d [g]
+  -> y

lsp/test_fixtures/f9_bom.n6

@@ -0,0 +1 @@
+@P x :: d [g]