feat: replace ad-hoc .improv parser with pest grammar

- Add improv.pest PEG grammar as the single source of truth for the
  .improv file format (v2025-04-09)
- Replace hand-written line scanner with pest-derived parser that walks
  the grammar's parse tree
- Add grammar-walking test generator that reads improv.pest at test time
  via pest_meta and produces random valid files from the AST
- Fix 6 parser bugs: newlines in text, commas in names, brackets in
  names, float precision, view name ambiguity, group brackets
- New format: version line, Initial View header, pipe quoting (|...|),
  Views→Formulas→Categories→Data section order, comma-separated items
- Bare names restricted to [A-Za-z_][A-Za-z0-9_-]*, everything else
  pipe-quoted with \| \\ \n escapes
- Remove all unwrap() calls from production code, propagate errors
  with Result throughout parse_md
- Extract shared escape_pipe/unescape_pipe/pipe_quote helpers, deduplicate
  hidden/collapsed formatting, add w!() macro for infallible writeln

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Executed-By: spot

src/persistence/improv.pest

@@ -0,0 +1,124 @@
+// ── .improv file grammar (v2025-04-09) ───────────────────────────────────────
+//
+// Line-oriented, markdown-flavoured format for multi-dimensional models.
+// Sections may appear in any order.
+//
+// Names: bare alphanumeric or pipe-quoted |like this|.
+//   Inside pipes, backslash escapes: \| for literal pipe, \\ for backslash,
+//   \n for newline.
+// Values: pipe-quoted |text| or bare numbers.
+
+file = {
+    SOI ~
+    blank_lines ~
+    version_line ~
+    model_name ~
+    initial_view? ~
+    section* ~
+    EOI
+}
+
+version_line  = { "v" ~ rest_of_line ~ NEWLINE ~ blank_lines }
+model_name    = { "# " ~ rest_of_line ~ NEWLINE ~ blank_lines }
+initial_view  = { "Initial View: " ~ rest_of_line ~ NEWLINE ~ blank_lines }
+
+section = _{
+    category_section
+  | formulas_section
+  | data_section
+  | view_section
+}
+
+// ── Category ─────────────────────────────────────────────────────────────────
+
+category_section = {
+    "## Category: " ~ rest_of_line ~ NEWLINE ~ blank_lines ~
+    category_entry*
+}
+
+category_entry = _{ group_hierarchy | grouped_item | item_list }
+
+// Comma-separated bare items (no group): `- Food, Gas, Total`
+item_list = {
+    "- " ~ name ~ ("," ~ " "* ~ name)* ~ NEWLINE ~ blank_lines
+}
+
+// Single item with group bracket: `- Jan[Q1]`
+grouped_item = {
+    "- " ~ name ~ "[" ~ name ~ "]" ~ NEWLINE ~ blank_lines
+}
+
+group_hierarchy = {
+    "> " ~ name ~ "[" ~ name ~ "]" ~ NEWLINE ~ blank_lines
+}
+
+// ── Formulas ─────────────────────────────────────────────────────────────────
+
+formulas_section = {
+    "## Formulas" ~ NEWLINE ~ blank_lines ~
+    formula_line*
+}
+
+formula_line = {
+    "- " ~ rest_of_line ~ NEWLINE ~ blank_lines
+}
+
+// ── Data ─────────────────────────────────────────────────────────────────────
+
+data_section = {
+    "## Data" ~ NEWLINE ~ blank_lines ~
+    data_line*
+}
+
+data_line = {
+    coord_list ~ " = " ~ cell_value ~ NEWLINE ~ blank_lines
+}
+
+coord_list = { coord ~ (", " ~ coord)* }
+coord      = { name ~ "=" ~ name }
+
+cell_value = _{ number | pipe_quoted | bare_value }
+
+number = @{
+    "-"? ~ ASCII_DIGIT+ ~ ("." ~ ASCII_DIGIT+)? ~ (("e" | "E") ~ ("+" | "-")? ~ ASCII_DIGIT+)?
+}
+
+bare_value = @{ (!NEWLINE ~ ANY)+ }
+
+// ── View ─────────────────────────────────────────────────────────────────────
+
+view_section = {
+    "## View: " ~ rest_of_line ~ NEWLINE ~ blank_lines ~
+    view_entry*
+}
+
+view_entry = _{ format_line | hidden_line | collapsed_line | axis_line }
+
+axis_line = {
+    name ~ ": " ~ axis_kind ~ (", " ~ name)? ~ NEWLINE ~ blank_lines
+}
+
+axis_kind      = @{ "row" | "column" | "page" | "none" }
+
+format_line    = { "format: " ~ rest_of_line ~ NEWLINE ~ blank_lines }
+hidden_line    = { "hidden: " ~ name ~ "/" ~ name ~ NEWLINE ~ blank_lines }
+collapsed_line = { "collapsed: " ~ name ~ "/" ~ name ~ NEWLINE ~ blank_lines }
+
+// ── Names ────────────────────────────────────────────────────────────────────
+//
+// A name is either pipe-quoted or a bare identifier.
+// Pipe-quoted: |Income, Gross| — backslash escapes inside:
+//   \| = literal pipe, \\ = literal backslash, \n = newline
+// Bare: no = , | [ ] / : # or newlines.
+
+name = _{ pipe_quoted | bare_name }
+
+pipe_quoted = { "|" ~ pipe_inner ~ "|" }
+pipe_inner  = @{ ("\\" ~ ANY | !"|" ~ ANY)* }
+
+bare_name   = @{ ('A'..'Z' | 'a'..'z' | "_") ~ ('A'..'Z' | 'a'..'z' | '0'..'9' | "_" | "-")* }
+
+// ── Shared ───────────────────────────────────────────────────────────────────
+
+rest_of_line = @{ (!NEWLINE ~ ANY)* }
+blank_lines  = _{ NEWLINE* }