Extract MA recursion and time-dim guard helpers #76

Context

Audit reports/audit-2026-05-18.md findings #2 and #3.

#2 — The MA recursion Phi_h = sum_{j=1}^{min(h,p)} A_j @ Phi_{h-j}
is implemented twice with different shapes:

• identified._ma_coefficients (src/impulso/identified.py:50-67):
  vectorised over (C, D) via einsum.
• identification.SignRestriction._check_restrictions_at_horizons
  (src/impulso/identification.py:209-225): single (n, n) draw
  inside the rotation Python loop, using @.

The math is identical; the two copies will drift when one is fixed.

#3 — _guard_no_time_dim is copy-pasted between IRFResult and
FEVDResult (src/impulso/results.py), differing only in the
"irf" vs "fevd" literal and the class name in the error message
(~17 lines duplicated exactly).

Resolution

Per plans/2026-05-18-audit-cleanup-plan.md (Decisions ledger Q6, Q7):

• New module src/impulso/_ma.py with one function
  compute_ma_phi(A: list[np.ndarray], horizon: int) -> np.ndarray.
  List-of-A_j signature. Numpy @ broadcasts over leading dims so
  the helper handles both batched (C, D, n, n) and single-draw
  (n, n) shapes without branching. Stacks with axis=-3 so the
  horizon axis lands just before (n, n) regardless of leading rank.
• identified._ma_coefficients splits B_draws into the A list and
  delegates. identification._check_restrictions_at_horizons splits
  B_draw similarly, delegates, then loops over horizons applying
  restrictions to Phi[h] @ candidate.
• _guard_no_time_dim lifts onto VARResultBase. Each subclass
  declares _PRIMARY_KEY: ClassVar[str] ("irf", "fevd"). Base
  helper reads it.

Acceptance

1. make check && make test clean.
2. No behavioural change visible to tests outside tests/test_ma.py —
   pure refactor.
3. tests/test_ma.py covers single-draw, batched, edge cases
   (horizon=0, horizon < n_lags, numerical agreement against the
   pre-refactor recursion on a fixed seed).
4. src/impulso/identified.py and src/impulso/identification.py
   each call compute_ma_phi exactly once; the old recursion bodies
   are deleted.

Shipping

PR3 of the 4-PR audit stack, stacked on top of #75 (PR2).