fix(:bcd): do not "inherit" buffer-local dir

Problem:
Buffer-local CWD (:bcd) is "sticky", similar to window-local CWD (:lcd).
But this contradicts one of its main benefits: per-buffer "project root"
for LSP, OSC7.

Other problems:
- A buffer created with :edit/:enew/:new silently inherits b_localdir
  (and b_prevdir) from the previous buffer.
- curbuf_reusable() refuses to recycle a scratch buffer that has
  `b_localdir`.
- After :new/:vnew/:tabnew the CWD sticks to previous buffer's
  `b_localdir` even though the new curbuf has none, so :new is not
  equivalent to ":split | enew", and getcwd() disagrees with
  haslocaldir().
- Requires "which buffer spawned this buffer" semantics that no other
  buffer-local state has.

Solution:
Drop sticky/inherit behavior of buffer-local CWD (:bcd).

- do_ecmd: always apply the new curbuf's dir (`fix_current_dir`), like
  `do_autochdir` already does. :tabnew from a :bcd buffer now reverts to
  global CWD (and fires DirChanged), same as :tabnew from a :lcd window.
- curbuf_reusable(): recycling a scratch buffer frees its b_localdir.

To get sticky/inherit behavior of CWD, use `:lcd`.

runtime/lua/vim/_meta/options.gen.lua

@@ -734,7 +734,7 @@ vim.o.cmp = vim.o.casemap
 vim.go.casemap = vim.o.casemap
 vim.go.cmp = vim.go.casemap
 
---- When on, `:cd`, `:tcd` and `:lcd` without an argument changes the
+--- When on, `:cd`, `:tcd`, `:lcd` and `:bcd` without an argument changes the
 --- current working directory to the `$HOME` directory like in Unix.
 --- When off, those commands just print the current directory name.
 ---
@@ -745,7 +745,7 @@ vim.go.cdhome = vim.o.cdhome
 vim.go.cdh = vim.go.cdhome
 
 --- This is a list of directories which will be searched when using the
---- `:cd`, `:tcd` and `:lcd` commands, provided that the directory being
+--- `:cd`, `:tcd`, `:lcd` and `:bcd` commands, provided that the directory being
 --- searched for has a relative path, not an absolute part starting with
 --- "/", "./" or "../", the 'cdpath' option is not used then.
 --- The 'cdpath' option's value has the same form and semantics as
@@ -1572,7 +1572,7 @@ vim.bo.ci = vim.bo.copyindent
 --- 		following occurrence.
 --- 							*cpo-~*
 --- 	~	When included, don't resolve symbolic links when
---- 		changing directory with `:cd`, `:lcd`, or `:tcd`.
+--- 		changing directory with `:cd`, `:tcd`, `:lcd`, or `:bcd`.
 --- 		This preserves the symbolic link path in buffer names
 --- 		and when displaying the current directory.  When
 --- 		excluded (default), symbolic links are resolved to

runtime/lua/vim/_meta/vimfn.gen.lua

@@ -960,8 +960,8 @@ function vim.fn.charidx(string, idx, countcc, utf16) end
 ---
 --- If {scope} is present, changes the current working directory
 --- for the specified scope:
----     "window"  Changes the window local directory.  |:lcd|
 ---     "buffer"  Changes the buffer local directory.  |:bcd|
+---     "window"  Changes the window local directory.  |:lcd|
 ---     "tabpage"  Changes the tabpage local directory.  |:tcd|
 ---     "global"  Changes the global directory.  |:cd|
 ---
@@ -3326,23 +3326,27 @@ function vim.fn.getcursorcharpos(winid) end
 --- is ignored.
 ---
 --- Tabs, windows and buffers are identified by their respective
---- numbers, 0 means current tab or window or buffer. Missing tab
---- number
---- implies 0. Thus the following are equivalent: >vim
+--- numbers, 0 means current tab/window/buffer. Missing {tabnr}
+--- implies 0 (missing {bufnr} does not; see below). Thus the
+--- following are equivalent: >vim
 ---   getcwd(0)
 ---   getcwd(0, 0)
 --- <If {winnr} is -1 it is ignored, only the tab is resolved.
 --- {winnr} is a |window-number| or |window-ID|.
+---
 --- If both {winnr} and {tabnr} are -1 and {bufnr} is missing the
 --- global working directory is returned.
+---
 --- Note: When {tabnr} is -1 Vim returns an empty string to
 --- signal that it is invalid, whereas Nvim returns either the
 --- global working directory if {winnr} is -1 or the working
 --- directory of the window indicated by {winnr}.
 ---
---- If {bufnr} is provided, {winnr} and {tabnr} must be -1 and the
---- working directory of that buffer is returned. An argument may
---- be -1 only if all preceding arguments are -1.
+--- If {bufnr} is provided, {winnr} and {tabnr} must be -1, then
+--- the buffer-local working directory is returned.
+---
+--- An argument may be -1 only if all preceding arguments are -1.
+---
 --- Examples of buffer usage: >vim
 ---       getcwd(-1, -1, 0)  " Get current buffer's directory
 ---       getcwd(-1, -1, 3)  " Get directory of buffer #3
@@ -4468,8 +4472,9 @@ function vim.fn.has_key(dict, key) end
 --- and {bufnr} has set a local path via |:bcd|, otherwise 0.
 ---
 --- Tabs, windows and buffers are identified by their respective
---- numbers, 0 means current tab, window or buffer. Missing
---- argument implies 0. Thus the following are equivalent: >vim
+--- numbers, 0 means current tab/window/buffer. Missing {winnr}
+--- or {tabnr} implies 0 (missing {bufnr} does not; see below).
+--- Thus the following are equivalent: >vim
 ---   echo haslocaldir()
 ---   echo haslocaldir(0)
 ---   echo haslocaldir(0, 0)