fix(ladder)!: every ramp step incl. AfterDays==0 becomes a Tranche, drop BuyNow

[cristim]

Every ramp step, including AfterDays == 0, is now a scheduled Tranche (immediate step: FireAfter == Now), and TrancheResult.BuyNow is removed. The immediate step used to be a BuyNow action that nothing persisted, so in-flight netting never saw it.

Breaking: the only consumer of BuyNow is the platform's internal/server/handler_ladder.go (cli and mcp checked); it needs a pin bump plus the platform B1 change (persist immediate rows, display-only plan JSON).

Tests: issue-input regression (gap 10, ramp [0:.5, 30:.5] gives two 5/hr tranches, step 0 at Now; fails before the change), nil InFlightUSDPerHour rejection. Units documented on Tranche.AmountUSDPerHour and InFlightUSDPerHour (#349).

Refs #36 (closes after the platform PRs).

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📥 Commits

Reviewing files that changed from the base of the PR and between 5b9155e and 829de8e.

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📒 Files selected for processing (6)

• CHANGELOG.md
• pkg/ladder/allocate.go
• pkg/ladder/allocate_test.go
• pkg/ladder/ladder.go
• pkg/ladder/ladder_test.go
• pkg/ladder/store.go

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[cristim]

Independent merge-gate review (Opus, claude-opus-5-5): APPROVE at 829de8e

Base: main 5b9155e (head not behind). Local runs from a fresh clone at the exact head, GOWORK=off, GOTOOLCHAIN=go1.26.9, GOMAXPROCS=2, -p 2.

Evidence

1. appendStep/buildTranche: every non-zero step becomes exactly one Tranche, in step order. Randomized golden + property harness (20,000 seeded cases: random valid ramps of 1-5 steps incl. fully-delayed, plus fixed sets {0.5,0.5}, epsilon overshoot {0.5,0.5000000008,1e-10}, sums just under 1 {0.4999999995 x2}, thirds, {0.004,0.996}; 0-3 allocations with random big.Rat gaps): per allocation the tranche amounts sum to the gap exactly (big.Rat), none dropped or duplicated, StepIndex strictly increasing, FireAfter == Now.AddDate(0,0,AfterDays) by == (AfterDays 0 gives FireAfter == Now, same instant and location, no epsilon). Golden vs base: base BuyNow (layer, amount, term, payment) is byte-identical to head's FireAfter==Now tranches, and all future tranches are identical apart from IDs (17,922 immediate items, 162k-line dump, cmp identical). No behaviour change for non-immediate steps. allocate.go change is comment-only, so Allocate output is unchanged.
2. go#36 scenario: gap 10, ramp [0:.5, 30:.5] gives two tranches 5 + 5, step 0 at Now. Fails before: head tests run against base code fail TestBuildTranches_ImmediateStepIsTranche ("Tranches count = 1, want 2") plus 6 updated step tests.
3. Deleted tests (rationale contents, data sources, small-fraction rationale): they asserted only the buy-now PlannedAction's step-rationale text and DataSources. The platform never reads TrancheResult.BuyNow (git grep BuyNow on platform origin/main: test comments only, handler_ladder_test.go:519,728). UI rationale and data_sources come from plan actions built from Allocation.Rationale/DataSources in assembleLadderPlan (handler_ladder.go:566-567), read by frontend ladder-timeline.ts:71,90 and ladder-timeline-editor.ts:17. What is lost: the per-step "ramp step i/n (x%): $a of $g total" string, never persisted or shown. Acceptable.
4. Consumers: cli and mcp origin/main have no BuyNow/BuildTranches/TrancheResult. No other go package references them. No stale BuyNow in README/docs.
5. Nil InFlight: TestAllocate_NilInFlightRejected passes on base and head (it pins existing behaviour; error contains "InFlightUSDPerHour must not be nil").
6. Units docs on Tranche.AmountUSDPerHour and InFlightUSDPerHour match plan amendment 6 (OD-equivalent $/hr until go#349, same unit as the gap).
7. CHANGELOG flags the break.
8. Tests: pkg, providers/aws, providers/azure, providers/gcp, ci_cd_sanity_tests go test ./... all exit 0; go vet ./ladder/ clean. Optional mutants (4, changed lines): drop the immediate step, FireAfter +1ns, duplicate the immediate tranche, FireAfter +1 day: all killed (7 tests fail each). git ls-files -z | xargs -0 git secrets --scan with the AWS patterns registered: exit 0 under both BSD and GNU grep. CI all green, mergeStateStatus CLEAN, labels match go#36, body says Refs ladder: buy-now ramp steps escape netting + follow-up hardening for L5 convergence #36.

Non-blocking follow-ups

• Stale doc comments: ladder.go:10-11 ("immediate purchase actions"), :19 and :24 ("buy-now purchase action and future tranche"); test comments ladder_test.go:149, 211, 797; TestBuildTranches_EmptyAllocations now checks len(result.Tranches) != 0 twice.
• CHANGELOG wording: the platform calls BuildTranches but never reads BuyNow, so it compiles unchanged on the bump. The behaviour change on bump is that immediate rows get persisted with FireAfter == run start, and that is B1's concern (grace bound vs cancelOverdue, #544).
• Plan T5/H5 randomized property test was not added in this PR. The harness above shows the property holds; adding it to the suite is a coverage follow-up.