feat: fastn_perf to record js usage

[siddhantk232]

fastn_benchmark_api and fastn_perf global object have been added
that are accessible in the browser console.

See fastn-js/BENCHMARK_README.md for the things you can do with these
objects.

The measurements are only recorded when you are on localhost or you
manually set window.FASTN_BENCHMARK. Some interesting counters are:

• mutable-created
• closure-update

Running this on a simple ftd counter app gives the following results:

• closure-updates : 1395
• mutable-created : 1392
• mutable-sets : 1395

This is pretty suspicious considering that the code only creates a
single mutable variable.

Running this on the design-system package, we get:

• closure-updates : 1121847
• css-cache-misses : 2
• css-creations : 2
• mutable-created : 1114812
• mutable-sets : 1118291

It is obvious that there's a lot of room for improvement and we can
hopefully shirnk these counter and get visible performance improvements
as well. I will have to study the js code (and the js output of ftd) to
learn more about why this happens.

Using the chrome dev tools, I also was able to figure out that there
exists a memory leak (memory grows periodically which is cleared by
forced gc). I couldn't trace the actual object that is getting
allocated. I will have to spend some more time on this.

Almost all of the code and docs in this commit is generated using
claude. It also generated a bunch of extra code that I deleted manually.
The deleted code included a test harness and related docs which is not
needed at this time and only contributed to noise. The code that is kept
is pretty minimal and a wrapper over the browser performance API. This
is good even when we change our js code drastically which is the goal
here.

The AI started with the following prompt:

I want you to understand fastn. This is a web framework. The fastn binary
spins up a webserver that takes request, reads corresponding .ftd file from
CWD and translates it into an html output that contains html/css/js.

There's also an option to build using fastn build this builds your current
project and outputs static html/css/js files in a directory. We are facing
performance issues with the js output, like resizing browser window is slow
among other things. I want you to look at ./fastn-js/js/**.js files, fastn-core
includes it all in a function called hashed_default_ftd_js.

all_js_without_test_and_ftd_langugage_js is the function that lists all the
js files that are included and the order in which they are concatenated. I want
you to help me create benchmarks for these js file contents. Later we'll do
performance enhancement and compare benchmark reports across changes.

Suggest all the ways to benchmark the js files.

Following this, most of the prompts were very small and mostly accepted
whatever Claude produced. In between, I manually cleaned some garbage
and fixed compiler errors. Asking claude to do all this was possible but
it was not worth it.

[siddhantk232]

This PR is not ready to merge yet. The next steps in my head are:

• Study and document existing js api. Preferably with jsdoc to get better IDE hints
• Reduce the API surface. We might have some functions that are unused. The fastn.webComponentVariable for example.
• Try to reduce the counters mentioned in this PR description.
• If it's impossible to shrink the allocations or the shrinking is not big enough, consider drastic changes like using preact/signals for our reactivity system.

This is a long task and I am doing this all with the help of Claude Code.

[amitu]

These findings are brilliant @siddhantk232, especially closure-updates : 1121847, wow! We def have a lot of scope of improvement here :-). Let's make fastn super awesome, I believe we may for good solve our backend issues by just addressing these. Or least make significant strides.