Fix four tax/CLI correctness bugs from code reviewVerified

1. Mixed FIFO lots were taxed entirely as long-term when the oldest consumed
   lot was long-term. fifo-sell now returns per-lot consumed chunks (acq-date,
   units, cost), stored on the realized record; us-longterm-gain allocates
   proceeds across those lots by units and counts only the >1-year slices. A
   10-old + 90-new share sale now reports the LT portion (700), not the whole
   gain (2500).

2. Net capital losses could reduce Canadian ordinary taxable income. compute-
   taxes now includes only 50% of POSITIVE net capital gains: ordinary income +
   0.5*max(0, cg). A loss no longer lowers tax (it carries over, unmodelled).
   Also clamp the US long-term-gain slice to >= 0.

3. ledger discovery had a dead fallback: the hard-coded Homebrew path made the
   bare "ledger" fallback unreachable and broke non-Homebrew installs. Now:
   the resolved `command -v ledger' path if found, else bare "ledger" for
   run-program to resolve on PATH at exec time.

4. `chirp plan --years` default (15) disagreed with its help text (25); both
   are now 30, matching optimize-plan's default.

Adds mixed-fifo-holding-period and capital-loss-not-ordinary regression tests.
114 checks pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>