feat(log): --serving-amount + --serving-unit flags for unit overrides

[phitoduck]

Summary

Adds the --serving-amount <N> + --serving-unit <unit> flag pair on
lose-it log, letting the CLI mirror the official UI's
display-unit-override behavior for cup/mL/fl_oz/tbsp/g foods. The
existing --grams N flag is preserved as a legacy alias that internally
rewrites to --serving-amount N --serving-unit g, so there is exactly
one code path through _build_log_payload.

Spec: docs/serving-unit-spec.md
(merged separately via PR #23).

Wire-byte change (FoodServingSize block)

For lose-it log "...soup" --serving-amount 490 --serving-unit mL on a
cup-measured food (native ord = 3):

Before — _build_log_payload with servings=2.07 (legacy --servings):

|27|2.07|1|28|3|1|1|2.07|
     ^      ^^   ^ ^
     f0     |ord |f4=1
            |    f3
            native ord (cup)

After — _build_log_payload with override params:

|27|2.0711088904878836|1|28|11|1|236.5882365|490|
     ^                   ^^    ^ ^           ^
     canonical_servings  |     | conversion  user input
                         |     f3=1.0        in chosen unit
                         chosen ord (mL)

The override path matches the spec's wire-evidence snippet
27|2.071...|1|28|11|1.0|236.588|490| byte-for-byte modulo
float-precision serialization (the universal cup→mL constant is
236.5882365 — the captured UI value 236.58800000000002 is a
3-decimal-place rounded copy of the same constant; the spec explicitly
tolerates the precision difference).

CLI surface

Invocation	Behavior
--serving-amount 490 --serving-unit mL	New override path
--serving-amount 490 (no unit)	exit 2, "must be passed together"
--serving-unit mL (no amount)	exit 2, "must be passed together"
--serving-amount 0 --serving-unit mL	exit 2, "must be positive"
--serving-amount 490 --serving-unit oz	exit 2, "ambiguous"
--serving-amount 490 --serving-unit mL --servings 2.0	exit 2, "mutually exclusive"
--serving-amount 490 --serving-unit mL --grams 100	exit 2, "mutually exclusive"
--grams 86 (legacy)	rewritten to serving_amount=86, serving_unit=g; same wire payload

Tests added

• tests/conformance/test_units.py (14 tests): resolve_unit dispatch
  (case-insensitivity, oz-ambiguity rejection, unknown-unit rejection),
  conversion_factor lookups (cup→mL ≈ 236.588, cup→g returns None,
  mL→cup reciprocal, diagonal entries = 1.0, grams special case = 100.0).
• tests/conformance/test_entries_serving_unit.py (8 tests):
  byte-compares the FoodServingSize block against the spec's wire
  evidence; regression-guards the per-serving FoodNutrients HashMap
  (no double-scaling); legacy-path regression guard for the existing
  --servings / --grams behavior.

Total: 93 tests pass, 5 skipped, 0 failed.

Deferred / out of scope

• Phase 3 of the spec — suggested alternatives on unit mismatch.
  The CLI currently exits with a clear unit_not_supported error;
  proactively probing the rest of the search results for an entry whose
  native unit supports the requested one is left as a follow-up
  (marked with # TODO(serving-unit-spec.md Phase 3) in cli.py).
  Rationale: the parallel --food-id work removes most of the need
  (the user can pin the right entry directly), so the suggestion table
  is no longer urgent.

Composition with the parallel food-id-flow PR

This PR adds only the unit-override flow; the companion --food-id
flow (PR #TBD, branch feat/food-id-flow) adds stable food references.
The two compose cleanly:

lose-it log --food-id <hex> --serving-amount 490 --serving-unit mL --meal snacks

Both PRs touch cli.py and entries.py, but in disjoint regions (this
PR adds new option params to log's signature; the food-id PR adds
--food-id next to query). A merge conflict is possible at the
import block; a 3-way merge will resolve it trivially.

Test plan

• uv run pytest --no-cov — all 93 tests pass, 5 skipped (live-only).
• uv run prek run --all-files — clean.
• tests/conformance/test_units.py added per spec Phase 1 case table.
• tests/conformance/test_entries_serving_unit.py added with
  byte-compare against the spec's wire-evidence snippet.
• Constructed-fixture wire-byte check confirms the FoodServingSize
  block emits |27|2.0711088904878836|1|28|11|1|236.5882365|490|.
• Live mutating log (deferred until after PR merges; the
  constructed-fixture byte-compare is the load-bearing verification).