engine: one Amount, not a ByCount and a ByAmount twin per verb

[ninthworld]

Goal

One Amount says how much, so a verb stops being duplicated once per way of counting and
Creatures you control get +1/+1 for each … becomes authorable without two more variants.

Context and evidence

Parent: #819, §2.2. First of two halves; #827 collapses the subject axis.

Effect carries the same verb three times over, once per way its number is derived:

Verb	Printed number	Count of permanents	Derived amount
pump a target	Pump	PumpByCount	PumpByAmount
deal damage	DealDamage	DealDamageByCount	DealDamageByAmount
gain life	GainLife	GainLifeByCount	—
lose life	LoseLife	—	LoseLifeByAmount
draw	DrawCard	—	DrawCardsByAmount
discard	Discard	—	DiscardByAmount
pump a class	PumpAll	—	—

Eleven variants for six verbs, and the table's holes are the point: LoseLifeByCount and
GainLifeByAmount do not exist because no M19 card needed them, so the next card that does
adds a variant plus an arm in each of ~6 exhaustive matches — including 86 arms in
sage-server/src/rules_text/words.rs and 86 Effect:: arms in
sage-server/src/rules_text/effects.rs. Effect is at 82 variants.

The codebase already solved this once, and its own reasoning is the argument.
CreateToken takes count_of as a field, not a create_token_by_count twin:

A field rather than a create_token_by_count twin, which is where this departs from
Effect::PumpByCount and its two siblings: a second variant would duplicate the four other
fields here … The field says where that number comes from; it does not add a second verb.

The counter-argument stated in PumpByAmount's docs is that two optional fields would make
"neither" and "both" authorable shapes that mean nothing. That is true of two Option fields
and false of a sum type:

enum Amount { Fixed(i32), PerPermanent { per: i32, count_of: PermanentCount },
              Derived { per: i32, source: DerivedAmount } }

Exactly one is present, by construction. CreateToken's field already proves the ergonomics.

The CR 608.2 semantics are identical across all three today — the amount is read once, as the
effect applies — which is what makes this a mechanical collapse rather than a behaviour change.

exclusions.json names one limit that is a limit of this shape: "Effects whose amount is
derived from a source outside the ones the IR names." Narrowing DerivedAmount is orthogonal
to this issue, but the entry should be re-read against the collapsed shape.

Scope

• One Amount sum type covering the three derivations already modelled.
• The eleven variants above collapsed to six verbs carrying it.
• Resolution reads the amount at the one CR 608.2 seam, once, exactly as each variant does now.
• The server's rules-text formatter renders the amount from one place instead of three arms per
  verb.
• Catalog JSON re-authored. No card's rules change.
• The validator keeps refusing an amount a given verb cannot mean.

Acceptance criteria

• PumpByCount, PumpByAmount, DealDamageByCount, DealDamageByAmount,
  GainLifeByCount, LoseLifeByAmount, DrawCardsByAmount, and DiscardByAmount no
  longer exist as separate variants.
• The four combinations the table is missing today — a life gain per derived amount, a life
  loss per counted permanent, and the two on the class row — are authorable with no Rust
  change, and at least one is proven by a test.
• Every affected card behaves identically; the existing behavioural suites pass with the
  re-authored catalog.
• The amount is still read once, on resolution, and a "this way" amount still reads the
  same resolution's earlier work (patient_rebuilding, scapeshift,
  liliana_untouched_by_death).
• nightmare_s_thirst still reads life gained this turn and not life gained after it
  resolves.
• make compat regenerated; make verify green.

Test and validation evidence

• Existing m19_* suites over re-authored JSON — this is behaviour-preserving.
• A new test for one of the previously impossible combinations, card-shaped.
• Rules-text tests for each verb's three renderings.
• make verify.

Dependencies and sequencing

After #820 and #822 so the catalog is correct first. Sequence before #827: the subject
collapse is much cleaner over verbs that already carry one Amount. Overlaps #824/#825 in
files — pick an order.

Estimated scope

Medium — focused cross-layer change: one type, eleven variants to six, and the formatter.

Risks and constraints

The three derivations have subtly different reading rules per verb — LoseLifeByAmount reads
of each named player ("half their life" is each seat's own half), while
GainLifeByCount counts relative to the effect's controller. Collapsing the carrier must
not collapse that distinction; it belongs to the PlayerRef/CountScope already present, and
a test per verb should pin it.

Non-goals

The subject axis (#827). Widening DerivedAmount to new sources. CreateToken, which already
has the right shape.

Documentation impact

docs/card-schema.md — the amount vocabulary. exclusions.json — re-read the derived-amount
entry. No ADR: this is the shape ADR 0003 already chose, applied consistently.