Aha!  The missing link between the source line numbering and the assembly code addresses
was to add the -g flag to generate stabs directives in the assembly .s file.

See https://www.danlj.org/mkj/lad/info/stabs.html for more details.

With that given, we now have:

Line 22863 et seq:
_SHOW_NUM:
;       .stabd  68,0,8291
LFBB2:
        pshs    u       ;
        leas    -5,s    ;,,
        leau    ,x      ; num, num
        stb     3,s     ; absx, absx
        ldb     9,s     ; absy, absy
;       .stabd  68,0,8297
        ldx     10,s    ; scale, scale
        lbne    L35     ;
;       .stabd  68,0,8303


The filename can be extracted from the last .stabs directive, eg:
;       .stabs  "source/main.c",132,0,0,Ltext2

- if the last .stabs filename was not the one we're interested in (i.e. main.c) then
don't output the code that follows the .stabd directive.

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In tailgunner.map:
          5A79  A$main$22792                       main.c

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In main.lst, lines 22970-22971:
   5A21                   22791 _SHOW_NUM:
   5A21 34 40         [ 6]22792         pshs    u       ;

Later in main.lst in "Symbol Table":
  4 A$main$22792       5A21 GR  |   4 A$main$22793       5A23 GR


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In main.rst, lines 22790-22792:
   5A79                   22791 _SHOW_NUM:
   5A79 34 40         [ 6]22792         pshs    u       ;
   5A7B 32 7B         [ 5]22793         leas    -5,s    ;,,              
In main.s, lines 22791-22792:
_SHOW_NUM:
        pshs    u       ;     

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In main.s, line 22791-22792:
_SHOW_NUM:
        pshs    u       ;

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In main.i, line 2294:
# 1007 "source/main.c"
static const int8 MINUS[] = {
  0 , ((int8)8), ((int8)4),
  (int8)-1 , ((int8)0), ((int8)14),
  0 , ((int8)-8), ((int8)4),
1 }; static void SHOW_MINUS(void) { Draw_VLp((int8 *)(MINUS)); }

static const int8 SCORE[] = {
  (int8)-1 , ((int8)0), ((int8)16),
  (int8)-1 , ((int8)12), ((int8)0),
...

In main.i, line 9579:  (corresponds with # 8291 "source/main.c"  (1007-2294+9579-1) {-1 for "# 1007 "source/main.c"})
static void SHOW_NUM(int16 num, int8 absx, int8 absy, uint16 scale) {

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In source/main.c, line 8291:
static void SHOW_NUM(int16 num, int8 absx, int8 absy, uint16 scale) { // left-aligned

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