feat: extend math engine expressions (#713)

* feat(math): add operator phrases, rounding, percentages, strip units, math functions

- Operator phrases: `to the power of`, `remainder of X divided by Y`, `divided by`
- Rounding directives: `to N dp`, `rounded`, `rounded up/down`, `to nearest X`
- Percentage extensions: `X to Y is what %`, `X is what % off Y`, `0.35 as %`, `X/Y as %`
- Strip units: `$100 as number`, `20% as dec`, `50% to decimal`
- Math functions: exp(), log2(), log10() via native math.js
- Scientific notation input: 1e5, 2.5e3
- Fix implicit multiplication breaking function names with digits (log2, log10)

* feat(math): add median, count aggregates and inline aggregate syntax

- Block aggregates: `median` and `count` keywords (like sum/average)
- Inline aggregates: `total of 3, 4, 7 and 9`, `average of X, Y and Z`,
  `count of X, Y, Z`, `median of X, Y and Z`

* feat(math): add phrase syntax for roots and logarithms

- `square root of X`, `cube root of X`, `root N of X`
- `log X base Y`
- `X is Y to what power` (reverse logarithm)

* feat(math): add extended trigonometry functions

- Degree trig: sind, cosd, tand, asind, acosd, atand
- Degrees as parameter: sin(90 degrees) → sin(90 deg)
- Hyperbolic inverse: asinh, acosh, atanh (native math.js)

* feat(math): add tau and phi constants

Both already supported natively by math.js, just adding tests.

* feat(math): add conditional logic and boolean support

- if-then-else: `if 5 > 3 then 10 else 20`
- Postfix if/unless: `42 if 5 > 3`, `42 unless 5 < 3`
- Comparison operators: ==, !=, >, <, >=, <=
- Boolean values: true, false
- Logical operators: and, or, &&, ||
- Boolean assignment and compound conditions

* feat(math): add fraction support

- `X as fraction`: convert decimal to simplified fraction (2/10 → 1/5)
- `X% as fraction`: convert percentage to fraction (50% → 1/2)
- `X/Y of Z`: fraction of value (2/3 of 600 → 400)
- `Z is X/Y of what`: reverse fraction (50 is 1/5 of what → 250)

* feat(math): add multiplier calculations

- `20/5 as multiplier` → 4x
- `50 as x of 5` → 10x
- `2 as multiplier on 1` → 1x (markup)
- `1 as x off 2` → 0.5x (discount)
- `50 to 75 is what x` → 1.5x (change)
- `20 to 40 as x` → 2x

* feat(math): add speed, energy, frequency, power and other unit extensions

New custom units: mph, km/h, knot, calorie, kcal, fathom, light year,
microgram, bushel.
Native math.js units verified: Hz/kHz/MHz/GHz, W/kW/hp, J/kWh,
GiB/MiB/KiB, deg/rad, ns/ms, nm/um/dm.

* feat(math): add reverse conversion syntax and shorthand unit conversion

- Reverse: `meters in 10 km` → 10,000 m, `days in 3 weeks` → 21 days
- Shorthand: `km m` → 1,000 m (two unit names show conversion factor)
- Verified: larger unit wins (1km + 1000m = 2km), area from mult (10m * 10m)

* feat(math): add rate calculations with natural language

- `per` keyword: `$99 per week` → $99/week
- `at` phrase: `30 hours at $30/hour` → rate multiplication
- `a day for a year` phrase: `$24 a day for a year`
- Math.js compound units handle rate arithmetic natively

* feat(math): add calendar calculations module

- days since/till/until: `days since January 1`, `days till December 25`
- days between: `days between March 1 and March 31`
- Relative dates: `5 days from now`, `3 days ago`
- Day of week: `day of the week on January 24, 1984` → Tuesday
- Week numbers: `week of year`, `week number on March 12, 2021`
- Days in period: `days in February 2020` → 29, `days in Q3` → 92
- Date offsets: `3 weeks after March 14, 2019`, `28 days before March 12`
- Current timestamp: `current timestamp`

* fix(math): humanize mc-prefixed units in compound unit results

$30 * 4 days now shows "120 USD days" instead of "120 USD mcday"

* fix(math): grouped numbers with unit suffix (1 000m → 1000m)

Grouped number regex now uses lookahead instead of word boundary at the
end, so "1 000m" correctly groups to "1000m" when a unit follows without
a space.

* fix(math): simplify implicit rate results (currency * time → currency)

$30 * 4 days now returns "120 USD" instead of "120 USD days".
When multiplying currency by time, the result is treated as an implicit
rate and the time unit is removed from the output.

Author: antonreshetov