Introduce a shared exact symbolic expression core - #1127

Stacked on #1083. This is the first reviewable implementation layer of #1126.

Outcome

This PR replaces the duplicated symbolic-expression implementations with one canonical exact DAG and carries those exact expressions through reduction-path composition. Asymptotic Growth is derived only when a terminal comparison or Big-O presentation needs it.

Canonical expression core

• adds problemreductions-expr as the single parser and expression implementation used by the runtime library and proc macros
• stores immutable nodes behind Arc, preserving shared subexpressions through substitution and node-table serde
• uses arbitrary-precision BigRational constants and validated owned symbols
• exposes only the semantic nodes Const, Var, n-ary Add/Mul, Pow, Exp, Log, and Factorial
• lowers subtraction, division, unary negation, and square root immediately into canonical operations
• normalizes operand order, coefficients, repeated powers, and neutral elements
• memoizes DAG traversals and batch Growth analysis by node identity
• deletes the duplicate proc-macro parser and legacy raw-overhead path

Exact path semantics

• composes every reduction overhead as exact symbolic expressions across the whole path
• does not greedily prune paths from intermediate Big-O projections
• converts exact terminal fields to Growth in one shared analysis context
• reports missing substitutions by output field and identifies the failing path edge
• distinguishes a valid one-node identity path from an invalid empty path
• uses one canonical identity-field syntax instead of repeated string field names

Explicit failure boundaries

• statically invalid parsed expressions such as 0 / 0, 0^-1, and log(0) return parse errors
• non-finite approximate results return ApproximationError; they never become successful infinities or O(1)
• unsupported asymptotic forms return serialized, deduplicated GrowthFailure reasons
• exact antichains are never truncated or silently widened
• missing variables and invalid factorial arguments remain explicit errors

There is no compatibility AST, parser fallback, antichain cap, or silent approximation fallback.

External oracle fixture

A committed fixture generated by pinned SymPy 1.14.0 checks:

• 50 exact cases: rational values, canonical free-variable sets, and polynomial classification
• 12 numerical cases: finite f64 results and the explicit overflow boundary
• 14 asymptotic-order cases from exact SymPy limits
• 8 factorial-domain cases

Regenerate it with:

uv run --project scripts python scripts/generate_symbolic_expr_fixture.py

Stack boundary

This PR supplies the canonical expression core and integrates it with the existing overhead/path APIs. The named SizeMap/SizeBound types, complete registry field accounting, and separate exact-bound search APIs remain later stacked changes in #1126.

Risk and rollback

This intentionally replaces the old expression representation rather than preserving compatibility. The main risk is semantic drift in normalization or path composition; the SymPy corpus, property tests, deep shared-DAG regressions, and full repository test suite cover those boundaries. Before later stack layers depend on this commit, rollback is a normal revert of this PR; no persisted schema migration is required.

Verification

• make check
• cargo test --workspace --quiet
• focused parser/domain/error tests
• 5,000-case dominance property test (0.47 s locally)
• independent fresh-context implementation review: approved with no remaining findings

All passed.

Coverage

• full instrumented run: 6,090 tests passed
• project line coverage: 95.81% (120,289 / 125,543)
• PR patch coverage: 98.39% (2,261 / 2,298), up from 96.26%
• changed-file coverage: expression DAG 99.37%, macro codegen 97.81%, pred-sym error boundary 100%