refactor!(formula): migrate parser to use pest

Replace the manual tokenizer and recursive descent parser with a PEG
grammar using the pest library.

This migration involves introducing a formal grammar in formula.pest and
updating the parser implementation to utilize the generated Pest parser
with a tree-walking approach to construct the AST.

The change introduces a stricter requirement for identifiers: multi-word
identifiers must now be enclosed in pipe quotes (e.g., |Total Revenue|) and
are no longer accepted as bare words.

Tests have been updated to reflect the new parsing logic, remove
tokenizer-specific tests, and verify the new pipe-quoting and escape
semantics.

BREAKING CHANGE: Multi-word identifiers now require pipe-quoting (e.g. |Total Revenue|) and
are no longer accepted as bare words.
Co-Authored-By: fiddlerwoaroof/git-smart-commit (gemma-4-31B-it-UD-Q4_K_XL.gguf)

crates/improvise-formula/src/formula.pest

@@ -0,0 +1,91 @@
+// Formula grammar for improvise.
+//
+// A formula has the form:   TARGET = EXPR [WHERE filter]
+// See parser.rs for the tree walker that produces a Formula AST.
+//
+// Identifier rules (bare_ident / pipe_quoted) mirror `bare_name` and
+// `pipe_quoted` in src/persistence/improv.pest: bare identifiers are
+// alphanumeric plus `_` and `-`, with no internal spaces; multi-word
+// names must be pipe-quoted.
+
+// Auto-skip horizontal whitespace between tokens in non-atomic rules.
+WHITESPACE = _{ " " | "\t" }
+
+// ---- top-level ----------------------------------------------------------
+
+formula = { SOI ~ target ~ "=" ~ expr ~ where_clause? ~ EOI }
+
+// The target keeps its raw text (including pipes, if any) — we capture
+// the span directly rather than walking into its children.
+target = { identifier }
+
+where_clause = { ^"WHERE" ~ identifier ~ "=" ~ filter_value }
+
+// ---- expressions --------------------------------------------------------
+
+// Used by parse_expr() — forces a standalone expression to consume the
+// whole input, so `1 + 2 3` fails instead of silently dropping " 3".
+expr_eoi = { SOI ~ expr ~ EOI }
+
+expr = { add_expr }
+
+add_expr = { mul_expr ~ (add_op ~ mul_expr)* }
+add_op   = { "+" | "-" }
+
+mul_expr = { pow_expr ~ (mul_op ~ pow_expr)* }
+mul_op   = { "*" | "/" }
+
+pow_expr = { unary ~ (pow_op ~ unary)? }
+pow_op   = { "^" }
+
+unary       = { unary_minus | primary }
+unary_minus = { "-" ~ primary }
+
+primary = {
+      number
+    | agg_call
+    | if_expr
+    | paren_expr
+    | ref_expr
+}
+
+paren_expr = { "(" ~ expr ~ ")" }
+
+// Aggregates with optional inline WHERE filter inside the parens.
+agg_call     = { agg_func ~ "(" ~ expr ~ inline_where? ~ ")" }
+agg_func     = { ^"SUM" | ^"AVG" | ^"MIN" | ^"MAX" | ^"COUNT" }
+inline_where = { ^"WHERE" ~ identifier ~ "=" ~ filter_value }
+
+// IF(cond, then, else).  Comparison is a standalone rule because comparison
+// operators are not valid in general expressions — only inside an IF condition.
+if_expr    = { ^"IF" ~ "(" ~ comparison ~ "," ~ expr ~ "," ~ expr ~ ")" }
+comparison = { expr ~ cmp_op ~ expr }
+cmp_op     = { "!=" | "<=" | ">=" | "<" | ">" | "=" }
+
+// A reference to an item.  `SUM` and `IF` without parens fall through to
+// this rule because agg_call / if_expr require a "(" and otherwise fail.
+ref_expr = { identifier }
+
+// ---- identifiers --------------------------------------------------------
+//
+// Mirror of improv.pest's bare_name / pipe_quoted.
+
+identifier = ${ pipe_quoted | bare_ident }
+
+// Backslash escapes inside pipes: \| literal pipe, \\ backslash, \n newline.
+pipe_quoted = @{ "|" ~ ("\\" ~ ANY | !"|" ~ ANY)* ~ "|" }
+
+bare_ident = @{
+    (ASCII_ALPHA | "_") ~ (ASCII_ALPHANUMERIC | "_" | "-")*
+}
+
+// ---- literal values -----------------------------------------------------
+
+filter_value = { string | pipe_quoted | bare_ident }
+
+string = @{ "\"" ~ (!"\"" ~ ANY)* ~ "\"" }
+
+number = @{
+      ASCII_DIGIT+ ~ ("." ~ ASCII_DIGIT*)?
+    | "." ~ ASCII_DIGIT+
+}