Hey everyone! We've been hard at work fixing bugs and improving native
codegen. Enjoy!

# Analysis

* Rename the class tag to extern in type functions
* Fix #2204
* Rework type unification in terms of subtyping.

This uses the new solver's understanding of subtyping to drive
unification for
some internal callsites. This is a broad change that should result in
more
    consistent inference.  For example:

    ```luau
    local function sum<T>(x: T, y: T, z: (T, T) -> T) return z(x, y) end
    local function sumrec(f: typeof(sum))
return sum(2, 3, function<X>(g: X, h: X): add<X, X> return g + h end)
    end
-- Prior we would claim that `b` is of type `add<X, X> | number`: not
good!
-- We now correctly claim that it is of type `number` thanks to bug
fixes
    -- made to subtyping that did not make their way to the unifier.
    local b = sumrec(sum)
    ```

# Native Codegen

* Improve vector handling in binary operations
* Fix missed optimization causing an assertion failure
* Propagate loads of STORE_VECTOR components produced by UINT_TO_FLOAT
* Constant-fold new operand of CHECK_BUFFER_LEN
* For register load-store propagation, use the second operand only for
LOAD_FLOAT
* TValue store cannot be removed when it's partially over-written
* Rework equality and ordering comparisons to fix issues and be more
type aware
* Migrate Luau NCG IrInst operands representation to SmallVector

# Runtime

* Fix 64-bit countrz to build on 32 bit Windows.
* Remap Luau local debug and type information after performing jump
expansion
* Add support for building Luau as a shared library
* Luau should be able to make fastcalls even if library name is obscured
by a local polyfill

# OSS Contributions

* #2163
* #2167

# Internal Contributors

Co-authored-by: Ariel Weiss <arielweiss@roblox.com>
Co-authored-by: Hunter Goldstein <hgoldstein@roblox.com>
Co-authored-by: Ilya Rezvov <irezvov@roblox.com>
Co-authored-by: Varun Saini <vsaini@roblox.com>
Co-authored-by: Vighnesh Vijay <vvijay@roblox.com>
Co-authored-by: Vyacheslav Egorov <vegorov@roblox.com>

---------

Co-authored-by: Hunter Goldstein <hgoldstein@roblox.com>
Co-authored-by: Varun Saini <61795485+vrn-sn@users.noreply.github.com>
Co-authored-by: Alexander Youngblood <ayoungblood@roblox.com>
Co-authored-by: Menarul Alam <malam@roblox.com>
Co-authored-by: Aviral Goel <agoel@roblox.com>
Co-authored-by: Vighnesh <vvijay@roblox.com>
Co-authored-by: Vyacheslav Egorov <vegorov@roblox.com>
Co-authored-by: Ariel Weiss <aaronweiss@roblox.com>
Co-authored-by: Ilya Rezvov <irezvov@roblox.com>
Co-authored-by: Ariel Weiss <arielweiss@roblox.com>