fix: make TemporalValue self-contained

[pdeveltere]

Problem

The published lib/types.d.ts references the global Temporal.* namespace:

export type TemporalValue =
  | Temporal.Instant
  | Temporal.ZonedDateTime
  | ...

That only resolves if the consumer's tsconfig includes the ESNext.Temporal lib, which exists only in TypeScript 7. Every consumer on TypeScript < 7 (the vast majority today) gets:

error TS2503: Cannot find namespace 'Temporal'.

This surfaced when mysql2 bumped its direct dependency to sql-escaper@1.5.0 — its tsc build now fails (sidorares/node-mysql2#4216, PR sidorares/node-mysql2#4392).

Fix

Type TemporalValue structurally, branded on Symbol.toStringTag — the same signal the runtime isTemporal check already uses:

export type TemporalValue = {
  readonly [Symbol.toStringTag]: `Temporal.${string}`;
  readonly epochMilliseconds?: number;
  toString(): string;
};

• No dependency on the global Temporal namespace → resolves on any TypeScript version, no lib or polyfill types needed downstream.
• Any real Temporal value (Instant, PlainDate, PlainTime, Duration, …) still assigns — verified against @js-temporal/polyfill.
• temporalToString's guard switched from 'epochMilliseconds' in value to typeof value.epochMilliseconds === 'number' (equivalent at runtime, narrows the optional cleanly).
• Dropped ESNext.Temporal from the build lib since nothing references the global anymore.

Existing tests (incl. temporal.test.ts) pass; build and lint are green.

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Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines          561       561           
  Branches       170       170           
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  Hits           561       561           

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