feat(core+cli): advanced financial goal projections

[kafkade]

Description

Builds the "advanced" layer of financial goal projections on top of the existing basic goal tracking, and exposes it through a new ldgr goals CLI command. All projection math lives in pure ldgr-core (no I/O, no floating point) and is deterministic.

What's included:

Core (crates/ldgr-core/src/goals/tracker.rs):

• Multi-scenario projection bands — ScenarioRates + project_band model an optimistic/expected/pessimistic annual-return range and return three projected completion dates instead of one.
• Inflation-adjusted targets — inflate_target, inflation_adjusted_target, and project_with_inflation grow the target as savings compound, so a goal preserves real purchasing power (real vs nominal target).
• Variable contribution schedules — ContributionSchedule (base monthly + step-ups + one-off lump sums) with project_schedule.
• Shared-budget allocation — allocate_budget divides a single monthly budget across competing goals by priority, deadline, proportional, or equal-split strategy, reporting per-goal shortfalls.
• Linked-account actuals — estimate_monthly_rate_from_history derives an empirical contribution estimate from real balance history.
• 19 new unit tests covering edge cases: zero/negative rates, past/absent target dates, unreachable and already-met goals, lump sums completing a goal, step-ups crossing the target month, allocation shortfall/surplus, and empty history.

CLI (crates/ldgr-cli):

• New ldgr goals command with list, project <id>, plan <id>, and allocate --budget subcommands, each supporting --output table|json.
• Goal definitions are read from ~/.ldgr/goals.json (interim source until vault persistence lands); current amounts are pulled from a goal's linked account balance in the vault when unlocked, falling back to a configured value.

Compounding uses a nominal monthly rate (annual / 12), matching the existing loans module, via month-by-month simulations bounded by a 100-year horizon so unreachable goals terminate cleanly rather than looping.

Related Issues

Closes #224

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Documentation update
• Refactoring (no functional changes)
• CI / infrastructure
• Other (describe below)

Component

• crates/ldgr-core/ — Core library (crypto, accounting, storage)
• crates/ldgr-cli/ — CLI tool
• crates/ldgr-server/ — Sync server
• bindings/swift/ — UniFFI Swift bindings
• apps/ios/ — iOS / iPadOS / watchOS app
• apps/web/ — Web app (Next.js + WASM)
• docs/ — Documentation

Privacy Checklist

• No plaintext financial data is sent to or stored on the server
• No new metadata exposure introduced (or documented if unavoidable)
• Any new external service interaction is documented in the trust boundary model — N/A, no new external service interaction
• Crypto changes use audited RustCrypto crates — no custom primitives — N/A, no crypto changes
• Key material is never exposed in error messages, logs, or Debug output

Checklist

• I have read CONTRIBUTING.md
• Tests pass (cargo test --workspace)
• Clippy passes (cargo clippy --workspace -- -D warnings)
• Formatting passes (cargo fmt --check)
• I have updated documentation (if applicable) — CHANGELOG.md updated; goal definitions documented via the command's built-in sample/help