feat(tariff): lab explains the sum — fee breakdown from the engine walk

"ALL 740 / 3h 2m" gave no derivation. explainFee in @parking/shared runs
the EXACT computeFee walk with an optional trace collector — one code
path, so Σ line items ≡ the amount by construction (golden V1 regression
byte-identical; instrumentation changes no fee). Items: contiguous
same-price increment runs (time window · N × unit · tier-card name),
window-package occurrences, stepped day totals (top-tier repeat
flagged), daily-cap clamps as NEGATIVE adjustments, entry grace.

/api/tariff/simulate returns `breakdown` (null when settled); the lab's
Outcome panel renders the lined table with a rounding note (raw min →
billed min at the increment — answers "why does 3h 2m bill as 4h") and
a total row. Works against active/historical versions and drafts alike,
so a night-package draft can be verified line by line before publish.
Largely delivers the wiki's open "composer price preview" item.

4 new engine tests pin the sum invariant + item shapes (97 shared green).

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/web/src/api.ts

@@ -762,6 +762,9 @@ export interface SimSessionPricing {
 export interface SimulateResult {
   currency: string | null;
   pricing: SimSessionPricing;
+  /** Line items explaining pricing.amountMinor (same engine walk, Σ ≡ amount);
+   *  null when the session is settled (within walk-back grace). */
+  breakdown: import("@parking/shared").FeeBreakdown | null;
   curve: { minutes: number; amountMinor: number }[];
   gracePeriodExitMin: number;
 }