fix(swift): honor --acronym-style for leading acronyms

[schani]

Bug

With --acronym-style set to anything other than the default (pascal),
Swift type names with a leading acronym word ignored the option. For
example, with --acronym-style original the JSON key FaqCoordinate
incorrectly produced:

struct FAQCoordinate: Codable {

while AFaqCoordinate (where the acronym "Faq" is not the first word)
correctly produced:

struct AFaqCoordinate: Codable {

Root cause

swiftNameStyle in packages/quicktype-core/src/language/Swift/utils.ts
calls combineWords with separate style functions for the first word vs.
the rest of a name, and separately again for acronym vs. non-acronym words.
The style used for a first-word acronym was hardcoded to
isUpper ? allUpperWordStyle : allLowerWordStyle, instead of being derived
from the acronymsStyle parameter that carries the user's
--acronym-style choice. The style used for a later-word acronym
(restAcronymStyle) already correctly used acronymsStyle. So acronyms
anywhere but the first word honored the option; the first word did not.

Fix

Changed the hardcoded firstWordAcronymStyle argument to use acronymsStyle
(matching how Kotlin's equivalent kotlinNameStyle already wires this up)
when isUpper is true:

-        isUpper ? allUpperWordStyle : allLowerWordStyle,
+        isUpper ? acronymsStyle : allLowerWordStyle,

Non-upper (camelCase, e.g. property names) first words are left as
allLowerWordStyle, matching existing behavior and JS/Swift lowerCamelCase
identifier conventions.

Test coverage

Added test/unit/swift-acronym-names.test.ts, which generates Swift output
directly via quicktype() for a type whose only property-bearing name is
FaqCoordinate and asserts on the emitted struct declaration name across
--acronym-style values original, pascal, camel, and lowerCase.

A unit test (rather than a JSON/JSON Schema fixture) is used because this
class of bug is invisible to the fixture harness: fixture tests round-trip
JSON through the generated code and diff the result, but the identifier
casing bug only affects the generated source identifier (the struct/type
name), not the JSON keys/values that get serialized and compared — the
mangled identifier is still self-consistent, compiles, and round-trips
correctly. This mirrors the precedent set by test/unit/java-acronym-names.test.ts
(added in #2851 for a related Java enum-constant acronym-casing bug), which
documents the same reasoning.

Verification

• Added the regression test first and confirmed it failed on unfixed code
  (3 of 4 acronym-style cases produced FAQCoordinate instead of the
  expected casing).
• Applied the fix; the regression test now passes (4/4).
• Ran the full unit suite (npx vitest run test/unit): 167/167 passing, no
  regressions.
• npm run build passes.
• Manually re-ran the CLI repro from the issue
  (node dist/index.js -l swift --acronym-style original input.json) and
  confirmed FaqCoordinate now renders correctly, while re-checking
  --acronym-style pascal (the default) still renders FAQCoordinate as
  before (no behavior change for the default style).
• The Swift fixture/compile tests (FIXTURE=swift script/test) were not run
  locally — the swiftc toolchain is not available in this environment.
  This will be validated by CI; note that fixture tests would not have
  caught this bug in the first place (see above), so the unit test is the
  operative coverage here.

Fixes #1599

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