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feat(en_aero): aviation context phraseology — altitude, FL, heading, squawk, runway, freq (#116)

Extends en_Aero_ICAO with the per-context reading rules from FAA AIM
4-2-9 and ICAO Annex 10 vol II, addressing the reviewer's point on the
upstream issue that 'not every aeronautical number is read the same
way'.

New methods on Num2Word_EN_AERO (call directly via the converter
instance — these are aviation-only and don't go through the
language-agnostic dispatcher's to= mechanism):

  to_altitude(value, unit='feet')
    < 10 000 ft: thousands as a single ICAO digit + 'thousand' +
      optional hundreds
        5500 → 'fife thousand fife hundred feet'
    >= 10 000 ft: thousands portion read digit-by-digit
        12500 → 'wun too thousand fife hundred feet'
        25000 → 'too fife thousand feet'

  to_flight_level(value)
    Always 'flight level' + 3-digit-by-digit code.
    230 → 'flight level too tree zero'

  to_heading(value)
    Always 'heading' + 3-digit-by-digit; pads to 3 digits.
    Heading 0 maps to 360 (north convention).
    030 → 'heading zero tree zero'
    360 → 'heading tree six zero'

  to_squawk(value)
    Always 'squawk' + 4-digit code; pads to 4. Validates 0-7777.
    7700 → 'squawk seven seven zero zero'

  to_runway(value)
    Accepts int ('27') or string ('27R', '09L', '36C'). L/R/C suffixes
    spoken as left/right/center; case-insensitive.
    '27R' → 'runway too seven right'

  to_frequency(value)
    Decimal point read as 'decimal'. String input preserves trailing
    zeros that float literals lose.
    121.5 → 'wun too wun decimal fife'

Tests: 17 new cases in tests/test_478_en_aero.py covering each method
including range validation, suffix parsing, and the FAA hundreds /
thousands altitude switch at the 10 000 ft boundary.

Sources cited:
  - FAA AIM 4-2-9 / faa.gov chap11_section_1
  - ICAO Annex 10 vol II
  - SKYbrary phraseology

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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