fix(timer): settle concurrent interval reads in order

[rekcilyssup]

Closes #49

Problem

The async iterator returned by ctx.interval(delay) stored a single pending
resolver. Each next() overwrote it, so:

• a timer tick could only settle the most recent read;
• earlier concurrent reads stayed pending forever;
• return(), throw(), and context disposal could only settle the latest read.

Fix

Replace the single resolver with a FIFO queue of pending reads:

• each timer tick resolves the oldest pending read;
• return() resolves every remaining read as done;
• throw() rejects every remaining read with the supplied reason;
• context disposal rejects every remaining read with the disposal error;
• ticks received while no read is pending are still dropped (pulse behavior preserved).

Tests

Added 4 tests covering: ordered settlement of concurrent reads, return()
settling all pending reads, throw() rejecting all pending reads, and context
disposal rejecting all pending reads.

Verified locally: yarn lint, yarn build, yarn test (167 passed).