refactor: remove version field & remove recursed flag

[kongxiangyan]

After reviewing the source code, I noticed that some parts of the logic do not appear to have any observable effect:

• The Recursed flag seems to be unnecessary.
• The cycle field (Link.version) also appears to be redundant.
• The isValidLink check inside propagate does not seem to be required.

After removing these parts, all existing tests still pass successfully, and the overall performance appears to improve.

However, I am not fully certain whether removing these pieces is entirely safe in all scenarios. I would appreciate guidance from the maintainers or reviewers on whether this simplification is acceptable.

This PR proposes removing the above logic to simplify the implementation and improve performance.

[Copilot]

Pull request overview

This PR refactors the reactive system by removing version-based link tracking and the Recursed flag, simplifying the change propagation logic.

• Removes the version field from Link interface and cycle global variable used for version tracking
• Removes the ReactiveFlags.Recursed flag (value 8) from the reactive flags enum
• Simplifies the propagation logic by removing the isValidLink() function and associated conditional branches

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/system.ts	Removes version field from Link interface, removes Recursed flag enum value, updates link() function signature to remove version parameter, simplifies propagation logic by removing isValidLink() function and version/Recursed checks
src/index.ts	Removes cycle global variable, updates all link() function calls to remove version/cycle parameter from effect(), effectScope(), computedOper(), and signalOper() functions

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[kongxiangyan]

bench before remove:

$ pnpm run bench

> [email protected] bench D:\Sources\forked\alien-signals
> npm run build && node --jitless --expose-gc benchs/propagate.mjs


> [email protected] build
> node ./build.js

Warning: disabling flag --expose_wasm due to conflicting flags
clk: ~0.07 GHz
cpu: AMD Ryzen 9 7945HX with Radeon Graphics        
runtime: node 20.17.0 (x64-win32)

| benchmark            |              avg |         min |         p75 |         p99 |         max |
| -------------------- | ---------------- | ----------- | ----------- | ----------- | ----------- |
| propagate: 1 * 1     | `847.44 ns/iter` | `825.98 ns` | `851.54 ns` | `892.48 ns` | `910.21 ns` |
| propagate: 1 * 10    | `  3.49 µs/iter` | `  3.43 µs` | `  3.51 µs` | `  3.55 µs` | `  3.55 µs` |
| propagate: 1 * 100   | ` 29.52 µs/iter` | ` 29.15 µs` | ` 29.65 µs` | ` 29.70 µs` | ` 29.97 µs` |
| propagate: 10 * 1    | `  6.53 µs/iter` | `  6.48 µs` | `  6.54 µs` | `  6.56 µs` | `  6.58 µs` |
| propagate: 10 * 10   | ` 33.29 µs/iter` | ` 32.94 µs` | ` 33.53 µs` | ` 33.57 µs` | ` 33.62 µs` |
| propagate: 10 * 100  | `289.67 µs/iter` | `277.90 µs` | `291.00 µs` | `319.50 µs` | `359.20 µs` |
| propagate: 100 * 1   | ` 62.79 µs/iter` | ` 61.28 µs` | ` 63.24 µs` | ` 63.42 µs` | ` 64.77 µs` |
| propagate: 100 * 10  | `325.02 µs/iter` | `314.40 µs` | `325.70 µs` | `359.90 µs` | `400.50 µs` |
| propagate: 100 * 100 | `  3.08 ms/iter` | `  2.98 ms` | `  3.09 ms` | `  3.72 ms` | `  4.16 ms` |

bench after remove:

$ pnpm run bench

> [email protected] bench D:\Sources\forked\alien-signals
> npm run build && node --jitless --expose-gc benchs/propagate.mjs


> [email protected] build
> node ./build.js

Warning: disabling flag --expose_wasm due to conflicting flags
clk: ~0.07 GHz
cpu: AMD Ryzen 9 7945HX with Radeon Graphics        
runtime: node 20.17.0 (x64-win32)

| benchmark            |              avg |         min |         p75 |         p99 |         max |
| -------------------- | ---------------- | ----------- | ----------- | ----------- | ----------- |
| propagate: 1 * 1     | `804.48 ns/iter` | `762.70 ns` | `819.51 ns` | `876.90 ns` | `894.04 ns` |
| propagate: 1 * 10    | `  3.32 µs/iter` | `  3.27 µs` | `  3.35 µs` | `  3.38 µs` | `  3.38 µs` |
| propagate: 1 * 100   | ` 28.08 µs/iter` | ` 27.60 µs` | ` 28.27 µs` | ` 28.45 µs` | ` 28.59 µs` |
| propagate: 10 * 1    | `  6.10 µs/iter` | `  5.92 µs` | `  6.18 µs` | `  6.26 µs` | `  6.28 µs` |
| propagate: 10 * 10   | ` 31.23 µs/iter` | ` 30.77 µs` | ` 31.42 µs` | ` 31.54 µs` | ` 31.68 µs` |
| propagate: 10 * 100  | `274.22 µs/iter` | `257.80 µs` | `274.60 µs` | `416.20 µs` | `636.90 µs` |
| propagate: 100 * 1   | ` 58.06 µs/iter` | ` 56.17 µs` | ` 58.35 µs` | ` 60.32 µs` | ` 63.45 µs` |
| propagate: 100 * 10  | `308.92 µs/iter` | `290.10 µs` | `310.50 µs` | `366.50 µs` | `560.60 µs` |
| propagate: 100 * 100 | `  2.90 ms/iter` | `  2.83 ms` | `  2.91 ms` | `  2.99 ms` | `  3.02 ms` |