`writeInt` incorrectly encodes numeric `-1` for 1-bit integers

[Gusarich]

BitBuilder.writeInt dangerously misencodes the numeric value -1 (JavaScript number) for 1-bit integers, resulting in silently corrupted serialized data. Instead of encoding -1 correctly as bit 1, it encodes it incorrectly as bit 0, causing runtime deserialization failures and severe data corruption.

Minimal Reproduction:

import { beginCell } from '@ton/core';

// Store numeric -1 in a single-bit integer (number, not BigInt)
const cell = beginCell().storeInt(-1, 1).endCell();
const slice = cell.beginParse();

console.log(slice.loadInt(1)); // Outputs: 0 (INCORRECT! Should be -1)

Cause:
The special-case logic for single-bit integers compares the original input directly to BigInt (-1n) without normalizing the numeric input (number), leading to incorrect encoding:

// Current broken logic:
this.writeBit(value === -1n); // fails when value = -1 (number)

Impact:

• Silent corruption: Negative values stored as number are incorrectly serialized.
• Severe consequences: Deserialization returns incorrect values, causing potential logic errors, invalid contract states, incorrect transfers, and financial loss.

Expected behavior:
Encoding numeric -1 as a single-bit integer (storeInt(-1, 1)) MUST produce a correct bit (1) identical to BigInt -1n.

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LLM Fuzzing discovery (see tact-lang/tact#3123)